The bottom line? It's coming out this fall with a beta available for developers today, and will be compatible all the way back to the iPhone 3GS (which is a bit of a surprise) and up to the iPhone 4 and iPhone 4S.
iOS 6 will also be available for 4th generation iPod touch devices and later. On the tablet side, iOS will be supported on the iPad 2 and the 2012 third-gen iPad. Sorry, original iPad owners. As you can see, there are a ton of new features in iOS 6 - you can read about all of the big ones on our iOS 6 page.
We'll have lots more news coming from WWDC this week, so stick around.