Right now, the one Internet source I trust about this issue is AnandTech. This website seems to have no agenda to vilify Apple or AT&T nor give them a pass on every problem. AnandTech also seems to know a lot about antenna problems on mobile phones.
So, after AnandTech's lengthy testing of the iPhone 4 with iOS 4.0, 4.0.1 and 4.1 beta, here are the results:
- Reception in average conditions is sometimes significantly better on the iPhone 4 than on the 3GS
- Signal strength is sometimes the same as or worse than the iPhone 3GS
- The iPhone 4 is better at holding onto calls and data at very low signal levels
I have two 3rd party cases, so I guess I'll just have to take another 3rd party case. I think the bumper is a waste of time if you have a real case. If you're worried about bulk, the Griffin Reveal is a great option. That's the case I'm using now. It is very thin and it has a clear backing.
Apple has also released iOS 4.0.1. While it doesn't fix the signal attenuation problem, it more accurately shows your signal strength and you shouldn't be able to make your bars go from five to one.
Apple also announced that a fix is coming for the proximity sensor issue in the next iOS update.