I reported this in one of the feedback threads for Beta 3, but I don't think it was picked up. Maybe it needs a thread on it's own.
The problem is simple, if you pick a res/refresh rate that your monitor can't handle (but your video card can) you get stuck with a black screen, with no way of reverting in-game. You have to go and edit your user.settings file.
The solution is one of the usual 'Click here in 15 seconds or I change back to the previous res' dialog boxes.