Here is a useful accessory for your iPhone 3G or 3GS, the Case Mate ID Holder iPhone Case.
The Case Mate ID Holder iPhone Case features a pouch on the back that lets you hold up to two credit card sized items, so you could carry your driving license and a credit card.
I think this is a great idea, and would be handy for when you just want to take your iPhone out with you and leave your wallet at home.
The Case Mate ID Holder iPhone Case comes in a variety of funky colors, including blue, red, green, black, silver, purple and pink.
If you want one they are available for $29.99 from Case Mate.