Apple released iOS 4.3 beta to developers. It’s available now in the iOS Dev Center. The beta is available for iOS devices and the second generation Apple TV. Supported iOS mobile devices are iPad, iPhone 4 and 3GS, iPod touch 3rd and 4th gen. Build number is 8F5148b. A pre-release version of the Apple TV software has been provided to test AirPlay functionalities with 4.3 apps.

It looks like iOS 4.3 has some cool new gestures built specifically for the iPad. 4 & 5 finger gestures; pinch to the home screen, swipe up to reveal multitasking tray, swipe left/right to switch apps.  
Update: even though Apple says the gestures are there, I can’t seem to be able to use them at all. They don’t work for me in Apple’s own apps, and there’s no option to activate them in the Settings. We’re hearing several reports of other users unable to use these gestures.

Other new features in this first beta of iOS 4.3 include a comeback of AirPlay streaming for video in 3rd party apps (opt-in for developers through API), Personal Hotspot , iAd fullscreen banners.