The dotMobi Mobile Web Developers Guide was linked to extensively a while back, and it’s a great document, definitely worth taking a look at. Vitamin recently published an article about developing a mobile-friendly site; it doesn’t cover the broad range of topics in the dotMobi guide, but it’s a good first take.
An interesting link I found in the comments to the Vitamin article was to this page about linking stylesheets for mobiles. I didn’t realize that a mobile web browser will not load subsequent style sheets once it encounters the CSS for mobiles. This can result in a notable bandwidth reduction, given the lengths that some style sheets can get to.