First we had issues with the iPhone 4 and its reception related to they way it was held. Now there are many users reporting a problem with the proximity sensor on the phone during a call. I have actually had the same issues with my new iPhone 4 as well. The problem occurs during a call when the phone is up against your ear, the sensor is supposed to turn off the screen. But users are reporting that the screen is staying lit thus causing accidental input.
The accidental input has caused many dropped calls, and calls initiated by mistake. Many users have reported this issue on the Apple support forums, but no solution has surfaced by Apple yet. There has been mentions of reboots or settings resets that fix the issue but no confirmation by Apple as of yet. Have you had this problem?