A careful perusal of the above keynote video from the iPad's world premiere highlights one of the gizmo's truly revolutionary features: a new set of gestures and interface improvements that may well signal what we'll see in future iPhone implementations.
First are the true multi-finger multitouch gestures: swipes, twirls, even long touches and drags. The whole spinning-the-doorknob moment in itself was pretty cool (and more than a little reminiscent of the pod bay door scene in 2001). Mind you, this is being done on a playing service that's considerably larger than that of a current iPhone — but if apps are being cross-developed to work on iScreens both great and small, it would make sense to replicate the user interface on both as well.
The various popover control and option panels are another great idea: select/change options, or pick from a list, without obliterating the entire screen. (And all hail the Media Navigator, an uber-popover that could make file browsers obsolete.) Again, an upgrade that makes the most sense on a larger screen, but there's no reason a similar functionality could be scaled down to iPhone screen size and still work.