GelaSkins has been quietly selling some funky iPhone skins for a few years now, but they've recently made the big jump to hard cases. You'll find all of the same high-quality art on their new line of cases for the iPhone 4S, iPhone 4, iPhone 3GS, and iPhone 3G. If you've got your own images you'd like to use, GelaSkins also has a web tool for putting your own graphics on the some accessories.
This is a sensible move to make, considering the iPhone 4 and iPhone 4S is more in need of impact protection than scratch resistance. but of course, you have to be willing to give up some of that slimness and unified design whenever slapping on a hard case. It's a tough balance to strike, and one I still haven't settled on personally.
They're a tad on the pricey side, but damn are they stylish.