With the developer release of iOS 6 Beta 3, both shared Photo Streams and VIP email features are now supported on the aging 3GS, and Apple has updated the iOS 6 information page to remove the previously listed restrictions.
It never made any sense to me why 3GS users weren’t getting these features. No one was going to buy a new phone to get VIP email, but if they knew they weren’t getting it, they may have a bad taste for Apple over it. The cost/benefit was hard to understand. Flyover and turn by turn feel like the sorts of things the older hardware can’t support, leaving out these other features though just felt like they were trying to get people to buy a new phone by screwing their customers — very un-Apple like.