Is your proximity sensor malfunctioning under iOS 4, leading your cheek to hang up on calls or otherwise frustrate your ability to have a conversation? iPhone's proximity sensor is supposed to detect when the iPhone is moved up and close to your face so it can dim the screen and turn off multitouch, preventing just those types of problems. With iOS 4, for some users, it seems to randomly stop working, making multitouch live again, and that's when the problems start.
Some folks who have been grappling with this issue say a clean restore fixed it. That means re-installing iOS 4 and setting up as a new iPhone -- not from backup. It can be an incredible pain in the apps, costing you time to re-enter settings and losing your place in offline games, but if face dialing and other proximity sensor issues are driving you even crazier than a re-install would, it's something to consider. Yes, even on a brand new iPhone 4. Seriously.
Sure, a magical iOS 4.0.1 update could show up any time to fix that antenna reception problem and cure proximity issues in one fell swoop... but it might not either. If the work involved in a clean install has you panicked, wait as long as your sanity allows and then get it over with as quickly and cleanly as you can. (Or explore Bluetooth and keep your phone in your pocket).
If you're experiencing the proximity problem, let us know. If you've done the clean install, let us know if that fixed it.