I've got a Commuter case on my new phone now. It's for the iPhone 4 and it's a universal case. It fits perfectly, camera isn't covered (not sure if it affects the flash, it doesn't seem to be), buttons line up, mute switch isn't blocked at all. I'm really impressed with it, and I had it before the phone got here, which I really wanted. It was $20 on amazon, in the pink/white color. Maybe the new 4S case design has some design improvements, but there's absolutely nothing wrong with the iphone 4 cases in terms of fitting.
Also, the case doesn't feel bulky at all and looks like it's pretty effective with protecting the phone. Definitely something to consider if you think a Defender design is too bulky.