I’ve written an article for ALA on CSS-based rollovers. It’s not as good as I would like it to be, but hey — I wrote an article for ALA!
(05/05 Update: I was pretty proud of this when it came out, but the article seems quaint in comparison to the techniques we’re using tody — image replacement, the pixy technique, sIFR — I wish I’d thought of all of them.)