There is another problem with cases, at least not the ones from Apple. I purchased a case for my iPhone 4 after searching everywhere for one. I got a very nice case but then my phone started to freak out. When I would make a call and move the phone away from my face to end the call or use the interface for something else, the phone would lock up. I could not do anything with it.
After going to my local Apple store, I was told that the case was covering up the proximity sensor that detects when the phone is moved away from your face. Instead of showing me the regular interface, I got a blank screen. Apparently Apple moved the proximity sensor from where it was in earlier models.
The part of the case that is covering the sensor is only about 3/8" from the top of the phone but apparently that is enough to mess with the sensor.
I'm thinking that this is probably why there currently are so very few cases available from any of the big manufacturers.