Incase!!!! Best case for the 3g iPhone, hands down.
I have a black Incase, silicon case. It fits VERY snug around the iPhone and doesn't pull away easily like the Belkin silicon cases do.
I also have an anti-glare screen protector (sold at the apple store), and carry the phone in my front left pocket by itself. Nothing else goes in that pocket ever. =)
Oh, and for the ladies.....
My girlfriend carries hers in her Dooney and Bourke purse, in the cell phone pocket. She has an anti-glare screen protector as well, and a pink Agent18 silicon flower case.