Though one of iLounge’s editors is extremely concerned about demagnetizing his credit cards through exposure of this type, and another is somewhat put off by the cheapness of the design, Case-Mate’s I.D.
Case ($30) for iPhone 3G or 3GS is a plastic shell in your choice of colors, complete with a hole in the back to fit two plastic cards of your choice.
You’re supposed to install a padding and protective sticker on the back of your iPhone — the “cheapness” element — then snap the shell on and carry your cards and iPhone together at the same time.