Okay, I've gone through 2 iphone cases, and I refuse to pay another $15 to $25 for another case until I know what you guys are using? I've tried the "rubber" glove type case first. This was great until the rubber became a little stretched, and when I put it in my pocket it was difficult to remove. Then I switched to a plactic clam style case. This was pretty nice until the clam started to come lose and the top piece kept coming off.
I use a "Royal Series" case from Gogo. It's one of those barely-there cases which offers protection for the back and the sides. I like it because I don't want to add any bulk to the phone and it goes in and out of the pocket pretty easily. But keep in mind that it doesn't offer the protection of a full-body silicone case.