Not sure if this has already been mentioned in this thread but is anybody suffering with the proximity sensor issues? At first I thought it was user error until it started to manifest itself more frequently. A quick visit to a few of the sites who regularly update news on the iPhone seemed to have confirmed this as yet another issue with the phone.
http://discussions.apple.com/thread.jspa?threadID=2475509&start=0&tstart=0
http://www.engadget.com/2010/06/29/is-the-iphone-4-having-proximity-sensor-troubles/
Personally, I've accidentally put a few people on mute or on speakerphone and have accidently hung up too. The screen can sporadically activate when the phone is next to your cheek, upon which contacts the screen and activates the call option buttons. It seems a random problem, sometimes it does it, other times it doesn't, there doesn't seem to be any specific condition in which it happens.
This along with the signal 'issues' have really begun to cheese me off with this phone now. I tolerated the signal issues but this has added insult to the injury. It has spoilt what I initially considered a highly anticipated release and for me has nullified any enjoyment of its new features.
For some reason I don't see a software fix coming soon for any of the aformentioned problems either.