Sencha Touch looks like an excellent toolkit for delivering HTML 5/CSS 3 mobile experiences. Check out these demos your mobile device to give it a test run: GeoCongress (makes use of geolocation), Solitaire (drag and drop and flip animations), Kiva (provides iPad popover controls).
Sencha Touch allows you to develop web apps that look and feel native on Apple iOS and Google Android touchscreen devices. It's built specifically to leverage HTML5, CSS3, and Javascript and makes use of HTML5 to deliver components like audio and video, as well as a localStorage proxy for saving data offline. Altogether, the entire library is under 80kb (gzipped and minified), and it's trivial to make that number even smaller by disabling unused components or styles.
Via @threefour