Ben Johnson’s got a nice little overview of some of the more clever animations found in some iPhone apps. These little touches are exactly the sort of thing that make iOS so great. I’ve always thought Tweetbot (and Convertbot, especially) are prime examples of apps that add fun little animations to take things over the top.
And though it’s rarely mentioned, I think sound effects can also be a key component of a highly-polished app. Of course not everyone has the sound up on their iPhone.