I had the same problem. I was listening to the radio and had locked the screen and buttons. Then I unplugged the head-set and couldn't unlock the N800 anymore. The power button worked fine, but the d-pad center button wouldn't work. After a moments panic, I replugged the head-set and the whole thing started working again. I'm not too sure about whether that was a coincidence or not with the head-set, but my advice is 1) not to panic, 2) wait for the screen saver to kick in again properly and 3) retry the unlock.
Good luck!