This is right on. I think we've seen everything that's 4S exclusive already and all that's left to see are new iPhone exclusives only. Obviously I hope I'm wrong, I'd love to see a few surprises for the 4S and older models, but it seems unlikely at this point.Video Record in the iPhone 3GS
Voice Control in iPhone 3GS
HDR in Camera in iPhone 4
FaceTime in iPhone 4
Siri in iPhone 4S
I'm sure there are a lot more, but right now this is what comes to mind.