It looks like while the older phones will be getting iOS 6, they won’t be getting all of the new features that come along with it. Apple has confirmed in the small print at the bottom of the iOS 6 preview page that FaceTime over cellular will be available for the iPhone 4S and the new iPad. It will not be available for the 3GS along with the Shared Photo Streams.
As for iPads, the first iPad won’t be getting iOS 6 at all, and the iPad 2 will not be getting Siri, while the new iPad (iPad 3) will have iOS 6. Similarly, the new Maps app that will be replacing Google Maps will be available for only the iPhone 4S, iPad 2 and the new iPad.
Other features that the iPhone 3GS will be missing out on include VIP list support, VIP/Flagged smart mailboxes, and Offline Reading List. These features will only be available for the iPhone 4 or later and the iPad 2 or later.