Apple has released iOS 4.3 beta 3. As this is beta version, so we advice that only developers play with these. iOS 4.3 is available for iPad, iPhone, Apple TV 2 and iPod Touch developers.
Prior to b3, Apple released beta 2, few days before.
Developers can download iOS 4.3 b3 firmware IPSW; those are part of iPhone Developer Enterprise or Standard Program.
Developers registered with Apple can access iOS SDK 4 for developing iPhone, iPod Touch and iPad applications.