Mobile Web Programming has put together a great list of UI guidelines for mobile platforms. The list includes iPhone, iPad, Blackberry, Nokia, Motorola, WebOS, Windows Phone, Samsung’s Bada and many more. Useful if you’re designing for one of these platforms, and also interesting if you want to compare different approaches.
UPDATE (13/1/11): Also see Guide to running 9 mobile platform emulators on your computer if you want to play with some of these UIs without purchasing a handset.
Link: UI Guidelines for mobile and tablet web app design (mobilexweb.com)