Preview at WWDC likely in June, followed by September launch.
iOS 6 Beta for Developers Now Available
The beta is available for the iPhone 3GS and later, iPad 2 and later, and the fourth-generation iPod touch. Builds of the new Apple TV software for second- and third-generation devices are also available.
In addition to iOS 6 releases themselves, Apple has also released iTunes 10.6.3 for developers, as well as Xcode 4.5 developer preview.