I have a couple issues with the proximity sensor. I wouldn't classify them as problems (it seems to work), but as unintended behavior that could be avoided by using and/or ignoring it.

The first is with Google Voice. When listening to voicemail using the earpiece, the proximity sensor is not used do disable the screen, resulting erratic behavior of the app, including interruption and/or repetition of playback. I found a this issue already listed on Google's site. Anyone interested in starring it (and hopefully raising its priority) can click here. If anyone has a fix I'd love to hear it but I posted this more the raise awareness (and hopefully) its priority.

The second has to do with extended dialing sequences. For example, I have a number like 123-456-7890,,,7745,,8848#, which pauses for a few seconds before proceeding automatically. If I place the phone to my ear before the sequence is complete, the sequence is halted (apparently the proximity sensor causes it to halt). If I move the phone away from my face, the sequence continues. Anyone have a fix for this?