Proximity sensor problem in iOS4

hi
i have a 3GS with OS4.0
i think i have a problem with the proximity sensor of my phone since i made the upgrade to OS4.
when im making or receiving calls i cannot use the button of the screen because it stays shut even when i take it away from my ear.

its very annoying because i cant answer a waiting call or enter the menu as i speak.
does anybody knows a solution for this problem?

thanks