Apple is adding Shared Photo Streams and VIP Mail boxes to the list of iOS 6 supported features on the iPhone 3GS. Previously, support for both of those iOS features were expressly excluded from the iPhone 3GS. Some eagle-eyes readers noticed, however, that Apple's iOS 6 page has just been updated to remove those exclusions.
Shared Photo Streams allows you to designate pictures to share over iCloud with friends and family, as well as to comment on, and read comments on those photos. (As long as those friends and family members are also running iOS 6 and/or OS X Mountain Lion)
VIP Mail allows you to set contacts whose messages you consider important enough to consolidate in a special email box, reducing the chances you'll miss them amid the clutter of spam, ham, and other inbox mayhem.
The iPhone 3GS hardware is over 3 years old now, so there will obviously be limits to the software it can run when compared to current generation devices like the new iPad or upcoming ones like the iPhone 5, which is expected to ship alongside iOS 6 this fall.
Apple also has a limited number of engineers, and those engineers have a limited number of hours they can spend optimizing new features for old devices, compared to creating new features for new devices (or we'll complain, come next keynote).
Binary compatibility itself is a huge feature -- the ability for the iPhone 3GS to run apps built for iOS 6. But all that said, given that Apple still sells the iPhone 3GS to this day, they should make an effort to get as many of the features that can run, to run.
Turn by Turn Navigation, 3D flyover Maps, Siri, FaceTime, and other, resource intensive or hardware dependent features won't be joining them, nor will Offline Reading List.
However, it's nice to see Shared Photo Streams and VIP Mail get the go ahead on the iPhone 3GS.
Sometimes Apple giveth.