Apple released iOS 4.3 today, 2 days ahead of schedule.  

Key Improvements and Changes 

1. AirPlay enhancements - in addition to music, movies, and photos, AirPlay now streams video from your Photos app so you can shoot a home video and instantly share it with everyone in the room. Even AirPlay-enabled apps and websites get the big-screen treatment with iOS 4.3. And if your photos are the feature presentation, you can play a slideshow with all the stunning themes available on Apple TV.
2. Faster Safari performance - the new Nitro JavaScript engine powering Safari runs JavaScript up to twice as fast as in iOS 4.2, meaning you get more speed behind each page load.
3. iTunes Home Sharing - Now you can play your entire iTunes library from anywhere in the house. If it’s on your Mac or PC, you can play it on your iPhone, iPad, or iPod touch over a shared Wi-Fi network. And not just music. Watch a movie or TV show. Play a podcast. Or listen to an audiobook. On whichever device you want — without having to download or sync. 
4. Personal Hotspot for iPhone 4 - now you can bring Wi-Fi with you wherever you go. Download iOS 4.3 on your iPhone 4, and the next time you find yourself without access to Wi-Fi but in 3G territory, enable Personal Hotspot and share your cellular data connection with your Mac, PC, iPad, or other Wi-Fi-capable device. You can share your connection with up to five devices at once over Wi-Fi, Bluetooth, and USB — with up to three of those connections using Wi-Fi. Every connection is password protected and secure. And it’s power friendly, too. iPhone detects when your Personal Hotspot is no longer in use and turns it off to save battery life.

Other enhancement include new Ping features (push notifications for comments and follow requests, post and like songs directly from the Now Playing screen, parental controls), new settings (messages setting for number of times to repeat an alert, iPad side switch setting to lock screen rotation or mute audio notifications and sound effects), and single tap conference call dialing with a pause to send a passcode.

How to Update to iOS 4.3

Updates are available for the iPad, iPhone (3GS and 4GSM), and iPod touch (3rd and 4th generation).  To update your device, connect it to iTunes and click the "Check for Update" button.  Direct links to download are also available from Apple, however we recommend using the automatic updating method through iTunes. 

Direct Download Links for iOS 4.3 (Official from Apple)

iPhone 4 (GSM)
iPhone 3GS
iPad 2
iPad
iPod touch 3G
iPod touch 4G
Apple TV 4.2 (8F191m)