I am having the same issue as well. It seems to happen to about 50% of calls I make. I lose all access to the screen and cannot end the call or go to speaker due to the fact the screen has gone black.
I hope this isn't a wide spread problem with the proximity sensor. I get my phone tomorrow so I will definitely check this out. Lets see how many people list this as a problem. If a lot of people complain about this it may be software problem that can be dealt with in an update. To soon to tell.