I have been working on a 3 column site which has given me some problems lately. I have looked all over and found some really cool examples. It wasn't until I came accross Position is Everything (p.i.e.) that I found the right answer for my situation.
Big John, the author of the site, has a great essay explaining the problems I was facing and some nice simple examples of code which allows me (and you) to over come these issues fairly easily. I was extreamly impressed by the "guest demo" by Holly Bergevin which shows just how complex one can get with valid css.