I can vouch for this, this exact thing is happening to my girlfriend. You can do a simple experiment to repeat this bug on your phone, you will need a test subject (with hair):
1) Go into a call (you can call your voice mail).
2) Put the phone right up to your subject, if your subject has long hair, you can go ahead and put the phone next to the subject's ear but **make sure that his/her hair is between the phone and his/her ear**. You can also just put the phone on top of your subject's head!
3) If you giggle the phone you will notice that the Droid's screen will periodically light up, indicating that it is unlocked and subject to accidental key presses. In normal operation the proximity sensor should keep this from happening. (NOTE: By "giggling" I mean, try and move the phone a few millimeters up and down and side to side.)
4) Now for the control part of the experiment : Try this same experiment up against your subjects ear **make sure there is no hair in between the ear and the phone**. You will notice that the phone screen stays dark, meaning it's locked, working as normal and not subject to accidental key presses.