my wife has the same problem too.. I had to install a screenlock app, which is annoying because she has to swipe the phone to unlock the phone just to end a call etc.. can't think of the name off the top of my head, will post later if it comes to me.
my wife had her droid replaced because of it and now the replacement is doing it too, when i had the droid never had problems. passed it on to her and then bam nothing but problems with the proximity sensor. not sure when it started so i can't say if it is related to the froyo update or not.
This has been talked about in the forums here before. After the update the proximity sensor seems to turn the screen back on when the phone tilts too far. As in, holding the phone between your head and your shoulder, or laying down with the phone to your ear. It's annoying and I wish they would fix it. I never had an issue until after the update to 2.2.