This is being discussed in a couple different threads. I'm not sure if there is a way to consolidate them but
here is another discussion.
After doing some experimenting, this is what I've found:
1) It never occurs inside or in darker environments. Covering/uncovering the sensor in the upper left corner makes the droid respond as expected.
2) It does not occur while on speaker phone. In this mode the screen goes blank after the user-specified screen time out and you have to hit the power button to turn it back on. Reading other posts/forums, this seems to be expected behavior.
3) Outside, sunny, bright environments are the real problem. The screen works fine, but as soon as I dial or answer a phone call the screen goes blank, even if I haven't held the handset up to my hear (the sensor never gets covered). Note though that this doesn't happen every time; I haven't been able to consistently recreate the blank screen issue yet. When it does occur, I have had some luck moving the screen around trying to get the sun shining directly at the sensor, but it's kind of a crap shoot.
4) Sliding out the keyboard causes the screen to come back on when the problem occurs.
On normal phone calls, it's not really a problem letting the other person hang up. The biggest issue is when leaving a voicemail or trying to check my voicemail. You can't end the message or enter your vmail password without sliding out the keyboard.
I hope that maybe narrows down the problem for some of you. Can you try it outside in full sun and see if you have similar results? Or maybe you get some ideas about where the problem lies so we can go about fixing it?
Cheers