Well my screen turns off when I use (hand hold) the phone, I believe there to be a proximity sensor that turns the screen off and back on when the call is complete. (No I haven't read the manual yet).

Therefore I'm guessing your noggin is too far away from the BB to trigger said sensor ....... Why not use a bluetooth headset if this is an issue? Holding your head like that is not healthy over a long period of time.
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