[junkiee]

first off, it's not the light sensor that is turning off the screen, it's a proximity sensor in the top left corner. second, the reason why it's doing it (and am only guessing) is that you are bringing the phone to your head before the phone makes a connection to the tower (it vibrates once it does if it set to do so) and the proximity sensor doesn't start doing it's job until the phone call has been initiated.

Try waiting for the connection to be made then put the phone to your head; bets are that the problem goes away.