This is a very interesting thread, but unfortunately not very helpful to me. I am having the same problem on a Samsung Moment, but not when I use it. It only happens when my wife uses it. After extensive troubleshooting, we have determined that it is because she wears shiny earrings and they reflect enough light into the sensors that even though the phone is up to her ear, the screen does not blank about half the time. This makes the phone completely unusable to her, as she is constantly putting people on hold or hanging up on them. The sensor is at the perfect spot for her earrings to cause this problem. Earring out, problem solved. Earring in, phone unusable.
Congratulations, Google. You've created a technical monstrosity that is unusable for over half the population because you failed to do even rudimentary testing on a completely unnecessary feature. After all, Palm fixed this problem in the early smartphone days by offering users a checkbox option to disable the screen when on a call. The screen was simply reenabled with a green "answer" or "dial" button press. What is wrong with that? Why do we need a stupid sensor that just makes things more complicated and doesn't even work all the time for the majority of folks?
Oh yeah, and the suggested recommendation to just quickly press the power button to disable the screen after dialing doesn't work on the Moment. It hangs up the call every time, no matter whether you press it immediately after dialing or after the person has answered.
Does nobody offer an app to disable the screen when on a call?