I recommend the iPhone Accessories forum for questions about iPhone accessories...
I'm happy with my Incipio Feather. I had an Incase Snap on my iphone4 which worked great as well, got it through the case program so it was the first design and doesn't fit the CDMA iphone4 or the 4S due to different mute switch position.
There are several other choices as well, see first line above; I think there are a couple existing current discussions on this very question.