Based on photos and details submitted by an anonymous source, we’ve compiled a collection of changes and improvements for your easy reference. Here’s what’s new and notable.

About. All of the portable iOS 4.3 devices show “4.3 (8f5148b)” as their version numbers; the second-generation Apple TV shows 4.3 (8F5148c) as its iOS version number.

AirPlay: Limited Web Video (And Other Video) Support Added. Introduced to iOS devices in iOS 4.2, AirPlay enabled near-realtime streaming of certain videos and photos to second-generation Apple TVs. iOS 4.3 expands AirPlay’s support to include streaming of certain web-based H.264 videos with AAC audio to the Apple TV, though the word “certain” turns out to be important here.

Notably, the web-based videos will not stream unless they are specifically authorized by the web sites’ developers to do so; iOS 4.3 looks for a permission tag before allowing the video to stream from the iOS device to a TV. This is almost certainly being done to placate movie or TV studios concerned that their content might be streamed from the Internet to television sets without their specific permission, and effectively blocks everything else from working unless individual web sites add the permission tags to each of their videos. While the effort required to add one tag is trivial, it remains to be seen how many sites will do so proactively, and across multiple videos. Audio from unauthorized videos will still stream over AirPlay, but the videos will only be watchable on the device’s screen.

Apple has also enabled self-made videos stored within the Photos application to stream automatically to Apple TV if you so desire. This was not available under iOS 4.2, and thereby meant that a video created on an iPhone 3GS or iPhone 4 would not be viewable on a TV unless it was transferred to a computer and resynchronized to the iPhone using iTunes, or sent to MobileMe or another online service from the iPhone and then viewed using a different app.

Camera + FaceTime. Code suggests that new camera filter effects will be added to certain devices, mimicking the realtime color and distortion tricks that the iPod nano 5G was capable of adding to videos. Apple has apparently changed the shutter noise slightly to a more metallic-sounding click, and the FaceTime application icon (on iPod touch, later on iPad 2, and in the Settings menu of iPhone 4) has been changed to a silver-clad version that’s fancier and similarly more metallic than the prior green version. It’s similar to the Mac icon for FaceTime, but with an iOS-shaped rounded square background.

Missed FaceTime outgoing calls are now indicated with a small upward-pointing arrow inside the prior FaceTime camera icon.

Find My Friends + Location Services. A new feature called Find My Friends appears to be in the offing for Apple’s MobileMe service, enabling users to instantly locate their iPhone/iPod touch/iPad-toting buddies (and family members)—if they want to be found. Apple has moved Location Services to a newly prominent position within the Settings menus of iOS 4.3 devices, most likely to make it easier for users to switch the feature on and off as privacy dictates.

iPad 2, iPhone 5 Versions Spotted. Code within the iOS 4.3 beta software shows identifiers for multiple versions of the fifth-generation iPhone and second-generation iPad, presently believed to represent distinct individual Wi-Fi, GSM, or CDMA iPads, and separate GSM or CDMA iPhones.

iPod touch 2G, iPhone 3G No Longer Supported. Unless Apple surprises everyone by releasing subsequent iOS 4.3 betas for the iPod touch 2G and iPhone 3G, this appears to be the end of the line for iOS updates for the 2008 devices. Prior iOS releases left them with fewer features than their successor products, as well as crippling slowdowns in the case of the iPhone 3G. Apple appears to be bidding them an unceremonious goodbye.

Notes Gets New Font, Loses Another. The iOS 4.0-vintage Chalkboard font has been ditched in favor of Noteworthy, a new version that looks vaguely like handwriting. Chalkboard represented a second and less polarizing attempt to offer a casual font for the built-in iOS Notes application; Noteworthy sacrifices a little legibility in favor of an even more relaxed and interesting-looking typeface.

Photo Slideshow Settings. A subtle change to Slideshows on the iPhone and iPod touch brings these devices into closer feature parity with the iPad, and seemingly expands the feature a little on the iPad as well. Previously, transition effects were selected from a list of five options in the iPhone or iPod touch’s universal Settings menu, and couldn’t be changed within the Photos application itself. Control over these transition effects was not available when streaming photos to a second-generation Apple TV; “dissolve” was the force-selected option.

Now, iPhone and iPod touch transition effects are selected solely within the Photos application—a change that initially doesn’t seem to be very different, as there are only five options to choose from, and they’re the same as before; this is just like the iPad’s prior Photos app and Slideshow button. However, the list of transitions automatically switches from “Cube, Dissolve, Ripple, Wipe Across, and Wipe Down” to “Ken Burns, Origami, Reflections, Snapshots, and Classic” when you’re playing photos over AirPlay through an iOS 4.3-equipped second-generation Apple TV. In effect, iOS 4.3 is enabling you to select from the additional transition effects the new Apple TV is capable of performing on its own, and eliminating the global transition settings in favor of ones that work on the specific screen you’re performing the photos through. The iPad also shifts between these two sets of options depending on whether it’s playing photos through its own screen or an iOS 4.3 Apple TV’s; earlier Apple TV software doesn’t allow these transition changes.

Updates of Apps. A small change to the App Store’s Updates tab now displays larger icons for each app awaiting an update, oddly omitting the version number and date of the update, and adding an odd “Installed” badge that isn’t actually a button—like the prior “>” button, it can be pressed to bring up the update page, where you can start updating the app.

AirPlay Improvements. In addition to being able to perform more types of iOS device-streamed video, including certain web videos, AirPlay now appears to be quicker at starting video playback and displaying photos.

iOS Software Number. It’s now listed as Apple TV Software 4.3 (2009.2) and iOS Build Version 4.3 (8F5148c), finally synchronizing what used to be two confusingly different numbers. Apple TV Software 4.3 now tracks with iOS 4.3.

New On-Screen Keyboard Design. Use of the Apple TV’s prior on-screen keyboards caused lots of grumbling due to seriously sub-optimal layouts that punished users of Apple’s included Infrared remote by requiring numerous left and right button presses to move around. Apple has redesigned the keyboard as a set of three six-by-seven keyboard grids, toggled using the remote’s Play/Pause button, significantly reducing required button presses. One keyboard is for lower case letters and numbers, the next is for upper case letters and numbers, and the last is for symbols.

Gestures: Home Screen, App Switching, Multi-Tasking Bar. For the iPad alone, Apple has added a four- or five-fingered gesture that enables users to return to the Home Screen by pinching the current application closed, mimicking the “close window” animation that normally happens whenever you hit the Home Button. Some users have speculated that this spells the end of the Home Button on future iOS devices, while others believe that it’s merely an alternative gesture to reduce physical wear and tear on the button.

New four- or five-finger left/right sweep gestures enable you to change iPad applications automatically, using a simple flat layer-shifting visual effect, rather than the 3-D swap effect used when switching apps with the multi-tasking bar. Additionally, a four- or five-finger upsweep gesture opens the multi-tasking bar at the bottom of the screen, removing the need to double-click the Home Button. These new gestures do not appear to be included on supported iPhone or iPod touch models. Developers are currently expressing serious concerns about these new gestures impacting their applications, suggesting that Apple enable a per-app or universal setting to disable them.

iPad 2 Camera and Screen Resolution. Art files within the iOS 4.3 beta have shown what appears to be a higher-resolution version of the shutter animation for the Camera application, designed to be displayed at 1024x768 or 768x1024 depending on the iPad’s orientation. These files appear to suggest that the second-generation iPad’s screen will remain at the same resolution as the first-generation model’s, and that the Camera application will use the entire screen rather than just a part of it. However, neither of these details is certain at this point.

Mute / Screen Lock Switch Toggle. iOS 4.1 removed the iPad’s physical Screen Lock switch, transforming it for unknown reasons into a Mute switch for alert sounds, a change that confused and angered many users. It was assumed at the time that Apple had made the change in anticipation of releasing a FaceTime-equipped iPad that would ring when calls were coming through, but that iPad has not yet been released, and current iPads probably don’t need an alert muting feature as much as a screen orientation lock.

iOS 4.3 adds a new Settings toggle that lets users choose to use the switch for either function. When the switch is used for Screen Lock, a Mute button appears to the far left of the multitasking bar. Change the switch to Mute and a Screen Lock button appears in the same spot instead. Problem solved.

Message Alert Tones. Under Messages, the former SMS application on iPhones, you can now choose to have a message alert tone play 1, 2, 3, 5, or 10 times in total, rather than the prior “Repeat Alert” feature, which said “if you ignore a message you will be alerted twice more,” providing only an on/off switch.

Personal Hotspot. Renamed from and replacing the prior “Internet Tethering” feature, Personal Hotspot enables certain iPhones in certain carrier-supported situations to share their cellular data connections with computers—and iPads. Turning Personal Hotspot on disconnects the incoming Wi-Fi connection of an iPhone. Personal Hotspot is indicated either with a double chain link icon, or with a purple Personal Hotspot: X Connection(s) bar at the top of the iPhone’s screen. Notably, an iPhone-connected iPad or iPod touch running iOS 4.3 will display the double chain link icon in place of its Wi-Fi icon. Also important: Personal Hotspot data can be shared over Wi-Fi, Bluetooth, or USB connections depending on the connected device; Wi-Fi was previously not supported.

You can use the randomly-generated Wi-Fi password created by the iPhone, or create a Wi-Fi password of your choice to protect the Personal Hotspot from unwanted use. Up to five devices can be connected simultaneously, with the current number of connections appearing on the iPhone’s screen. It’s worth mentioning that the Personal Hotspot feature was first revealed as a Verizon CDMA iPhone option at Verizon’s iPhone event, and appears in as-yet-unreleased iOS 4.2.5 for that device; it will not likely come to other iPhone versions before iOS 4.3 is released. The predecessor version of this feature remains under the name Internet Tethering in prior iOS versions, and is limited to a single device connection over Bluetooth or USB, then only in countries that permit support for the data-draining feature.

We’ll update this article with additional details as we receive them. iOS 4.3 is expected to be released to the public some time in the next one to three months. Thanks to our source for the screenshots and details.