Earlier today, Apple made the iOS 4.3 available to its customers via iTunes, two days ahead of its previously announced March 11th release date.

iOS 4.3 is the release that will ship with the iPad 2, but it will also work with the iPhone 3GS, the AT&T iPhone 4, the third- and fourth-gen iPod touch, and the original iPad. If Apple sticks to its normal summer release schedule for new iOS versions, 4.3 will likely be the last major revision to iOS 4 before iOS 5 comes down the pike.

With this update, Apple has pulled the plug on iPhone 3G and second-generation iPod Touch users – rather than giving them a version of the 4.3 release stripped of its more performance-intensive features, the company has simply declined to issue iOS 4.3 for these devices at all. Also missing from the support list is the Verizon iPhone, though I suspect that this will be rectified through either a separate iOS 4.3 build or perhaps a 4.3.x build at some point in the near future.

The end of support for older devices isn’t exactly surprising, though I do wish that Apple had at least waited until iOS 5 to completely drop support for the slower hardware – Apple was selling new second-gen iPod Touches as recently as September 2010, and it continues to sell them refurbished as of this writing. All things must pass, but to cut off such recent customers seems a bit harsh. Still, Apple’s willingness to be the bad guy in this instance does help to prevent the hardware and software fragmentation endemic to the Android platform.

Older hardware aside, this article will focus mostly on what iOS 4.3 brings to users of existing devices. This is partly because the iPhone 3GS and first-generation iPad are what I’ve got to play with, and partly because most of the iPad 2-exclusive features relate to FaceTime, which by now is a thoroughly known quantity for iOS users.

A major component of the update is improved JavaScript rendering speed, courtesy of the same Nitro engine found in the desktop version of Safari. Apple says the new engine is about twice as fast as the old one, but is it true?

Sunspider isn’t everthing, but as a standard benchmark it’s a useful tool for measuring relative performance - relative to iOS 4.2.1, iOS 4.3 is actually a bit more than twice as fast at running JavaScript.

This is a welcome and impressive improvement, especially for those of us who aren’t using Apple’s latest and greatest. The Motorola Xoom is included for comparison, and while JavaScript performance isn’t quite as good on the original iPad, iOS 4.3 does a lot to close the gap, especially given the iPad’s older internals.

In practice, I wouldn’t say there’s a night-and-day difference, but pages seem to load with a bit more pep. No, it’s not a scientific analysis, but you can rest easy knowing that iOS 4.3 isn’t going to slow your device down while you’re on the Web. 

Another major addition is Personal Hotspot WiFi tethering for the AT&T iPhone 4, a feature introduced in iOS 4.2.5 with the Verizon iPhone 4. The AT&T phone can share its 3G connection with any 3 WiFi devices (down from 5 in the Verizon version, though this is almost definitely a carrier choice rather than a hardware limitation), but in order to use it you’ll need to shell out for the “DataPro” data plan that offers 4GB of bandwidth a month for $45. Sorry, iPhone 3GS users, but this feature won’t be available for you.

iTunes streaming also comes with the 4.3 release – once everything is properly configured, you can stream music or movies from your computer directly to your iDevice from your computer’s iTunes library. This is an extension of the Home Sharing feature that debuted with iTunes 9 (though iTunes 10.2 is required for this particular implementation).

If you haven’t already, you’ll first have to setup Home Sharing in iTunes using your Apple ID – this Apple ID will have to be the same as the one you use for your iOS device. Then, on your iOS device, you can find the Home Sharing settings in the settings for the iPod app.

Once you’re setup there, you should be able to stream from any Home Sharing-enabled computer on your network. In my admittedly limited experience with the feature, I had no problem sharing my iTunes library with either my iPhone or my iPad over my wireless network. It gives my 16GB devices the ability to see everything in my much-bigger-than-16GB iTunes library, which means I can finally have full access to all the music on my desktop by hooking my iPhone to the speakers in the kitchen. 

Those are probably the biggest functional changes, but there are some smaller things of note, most of which are best demonstrated with screenshots.

A few of the more user-facing: a new Noteworthy font in Notes app joins Helvetica and Marker Felt:

The Location Services menu has been moved to the top level of the Settings menu, giving you access to all of your Location Services-enabled apps:

The iPad’s switch, turned into a mute button in iOS 4.2, can once again be used as an orientation lock.

If you set the switch to work as an orientation lock, you can now find a mute button in the multitasking bar where the software orientation lock used to live:

And, of course, there’s a slew of even smaller changes: users can now delete an app which is currently downloading instead of having to wait until it finishes downloading, the iPhone now vibrates twice for text messages, and there are plenty of bugfixes that you can read about in the release notes.

Still MIA is an improved implementation of the AirPrint feature, introduced in iOS 4.2, which was originally intended to allow iOS users to print to any printer shared via iTunes by a PC or Mac. This feature was scaled back at the eleventh hour to support only direct printing to a handful of mostly-new printers built to support the feature. Workarounds exist to get it working with any printer, but official support for any ol’ printer has never materialized, and Apple has never offered much of an explanation.

Developers and/or tinkerers can also use XCode to unlock some iPad touch gestures that may be candidates for inclusion in the next iOS. These gestures use four or five finger swipes to reduce the number of times you have to quit what you’re doing to poke at the Home button: you can pinch to get to the home screen, you can swipe upward to see the multitasking bar, and you can swipe left or right to navigate between open apps.

Buried or not, these gestures could tell us something about possibilities for iOS 5: These functions look to enhance the existing iOS multitasking experience, but not necessarily to replace it with something else. It’s possible that Apple will look at Android and make some UI changes based on what Honeycomb does well, but if these gestures can be taken as an indication of things to come, we may not see any iOS UI overhauls when iOS 5 is unveiled later this year.

iOS 4.1 brought much-needed stability and performance improvements to the original release; iOS 4.2 brought iOS 4’s features to the iPad, which arguably needed them even more badly than other iOS devices; iOS 4.3 is polishes the original iOS 4 release to a dull sheen without changing anything fundamental to the experience.

For those of you with supported devices, there’s basically no reason not to download the iOS 4.3 update. You might not notice or use each and every one of its enhancements, but it’s certainly not going to break anything. This is just about as mature as iOS 4 is going to get - we may see the odd update to fix security or stability issues or possibly to bring the Verizon iPhone 4 into the fold, but this is likely the end of the line. If you don't like what you see in iOS 4, your best bet is to wait patiently for iOS 5, which we'll probably see more of later this spring.