One thing that's interesting: if Apple intended to stop sales of the iPhone 3GS when the iPhone 5 ships, you might expect them to eliminate the iPhone 3GS from the upgrade list for their next-gen iOS.
Since the device is already free under contract, could Apple be planning to make the device ridiculously inexpensive, to be able to sell it prepaid on the cheap? We'll see, this fall.
Siri will be available only on iPhone 4S and the new iPad (or the third generation iPad), but that is less about processor or hardware, and more about not stressing the Apple servers that must process verbal queries.
Shared Photo Streams requires iOS 6 on iPhone 4 or later or iPad 2 or later, or a Mac computer with OS X Mountain Lion. An up-to-date browser is required for accessing shared photo streams on the web.
FaceTime will support cellular in iOS 6. That feature also requires an iPhone 4S or new iPad. Yep, you can't use FaceTime on cellular with the iPad 2 or iPhone 4.
VIP list and VIP and Flagged smart mailboxes will be available on the iPhone 4 or later and iPad 2 or later.
Offline Reading List will be available on iPhone 4 or later and iPad 2 or later.
Finally, Made for iPhone hearing aids require an iPhone 4S.
We expect more limitations for the eldest iPhone in Apple's stable, the iPhone 3GS, to be found when iOS 6 ships.