I actually like the proximity sensor, it keeps me from hanging up or starting all sorts of stray apps with my cheek. However, yes it is way too sensitive to ambient light and has lead to many games of peek-a-boo with the phone blinking in and out while I try to stab the Accept or End buttons.

What I have resorted to doing is when I want to answer or hang up the phone I slide the keyboard open. This disables the proximity sensor and leaves the screen active. Then when I bring the phone up to talk I slide the keyboard closed again to reactivate the sensor. Because there is another delay as the orientation changes I leave the phone locked in landscape mode; it's easy enough to operate sideways that this doesn't bother me that much. Still, this is a Micky mouse way to have to deal with badly designed hardware.