I'm using the new beta driver (for a week or so (er whenever it came out)), and haven't had any black windows yet... but I haven't really pushed things either.
Did you ever notice that there is an art to getting the the black window contents to appear? Like, resize the window just to the edge of the video memory limit and it might show you the image? Or maybe you've tried "Minimize" "Maximize!" That's bound to clear up some MBs? Right?? Or perhaps you've tried the ol' Ctrl-Alt-F1, Ctrl-Alt-F7 to see if switching the session will refresh things? There's endless possibilities when you use your imagination!