Jakob Nielsen’s most recent Alertbox has some interesting findings about mobile site design and usability:
Summary: Good mobile user experience requires a different design than what’s needed to satisfy desktop users. Two designs, two sites, and cross-linking to make it all work.
One interesting tidbit: “This inequality [between desktop and mobile UI] is symmetric: mobile users need a different design than desktop users. But, just as much, desktop users need a different design than mobile users” (emphasis in original).