Confused proximity sensor?

Question

my iphone 3GS is doing the opposite of what it's supposed to do: when i hold it to my ear, the screen stays lit, and when i take it away, the screen goes black.
this has two negative consequences: one, my ear tends to press random "keys" on the screen, and when i take the phone away from my face to hang up, the screen goes blank and unresponsive. (i can sort of work around this by putting my thumb over the sensor, which activates the screen - exactly the opposite of what it's supposed to do - and then press a button.)
it's possible that this is being caused by my screen protector, although i can't imagine how or why (and i'm not the only iphone user with a screen protector), or by something wrong with the phone. (faulty sensor or firmware bug, etc.)
has anybody else had this problem, or more importantly, found a fix for it?

Answer 1

i am using a (very nice) speck skin-pro case, which i thought was 3gs compatible - guess it's not. until i can order a new case, i'll just cut a larger hole in this one.
i would have expected the proximity sensors to not work at all if they are covered in black rubber, but instead they sometimes they actually work backwards - with a call active, and the phone away from my face, the display blacks out, but the display turns on when something (face or thumb) gets close. very strange.
but this at least allows for a bit of a workaround: to hang up calls or push other buttons, i stick my thumb over the sensors, which turns the screen back on.