Can you give us more details? is the phone in your pocket, or is it sitting on a table?
Do you see it happening, etc.?
My first thought would be the camera button.
EDIT: Now that I have some more time to elaborate, there's a setting under settings > applications > photos+camera where it enables you to press and hold the camera button to wake up the phone.
If the box below that is unchecked, it means the phone will not check to see if something is blocking the proximity sensor. I leave it unchecked because I prefer the speed and I don't pocket press the button. But it may be the cause.