I've used iSkin Solo cases on all of my iPhones (3 now). They're thin, so they don't make the phone too bulky, but they add more than enough protection to keep the phone safe from reasonable drops (like arm height onto the floor, not down 3 flights of concrete steps). The cases hold up pretty well too, I had my 3GS for over 2 years, same case the entire time, and the case and the phone look just like the day I got them.
I also put a screen protector on them to keep the screen from getting scratched up, that usually needs to be changed every 6 months to a year. Not bad considering I just about always have my phone in my pocket, even out in the shop, or splitting wood, or even in my uniform pocket under my turnouts on fires, vehicle accidents etc at work.
