There's an open trac ticket for this already - see
here. As I just posted there, you don't have to reboot - moving from windowed to fullscreen and back fixes it - and there may be a simpler way of repairing the damage (backslash key/permanent windowed mode), I just haven't tested it.
What I notice is that I'm running on ION, plus at least one of the posters here is as well. How about the others? Is this is a general problem, or an NVidia-specific one?
What's interesting is that it appears on Linux and Windows, it seems, so that suggests it's the code and not the hardware or drivers...
After that, we need some guidance from someone as to where to look next.
EDIT
Okay, just did some quick testing... it seems to be related to calibration of overscan.
I was getting the issue in fullscreen (which I'd calibrated) but not windowed (which I hadn't). However, I can move the problem to windowed mode by simply calibrating that as well, and I can remove it (albeit with a large chunk of my screen image...!) by resetting fullscreen to (0,0,0,0).
Toggling between fullscreen and windowed (even if you say N to "would you like to keep this resolution?") remains enough to fix it, but backslash doesn't seem to (although that may be my configuration since I'm starting in fullscreen and thus may not be able to toggle back). Running in calibrated window mode permanently doesn't seem to wash since the calibration settings aren't persistent (again, maybe something I'm doing wrong, don't quote me on that).
