I hate the OtterBox cases and whatnot with a passion, but I'm pretty impressed with the casemate case [1] for my 3gs. Two years later and the back is still shiny; many drops and not a crack. Best of all, it doesn't make the phone any bulkier or affect its profile.
[1] http://www.case-mate.com/iPhone-3G-Cases/Case-Mate-iPhone-3G...