Me Too! I have a nearly new X61 Multi-Touch Tablet running XP SP2. In my case it seems to happen while I'm clicking on various Windows and the screen darkens to a dark gray, although if you look closely you can still see the Windows through the dark gray. Attempts to lighten the screen do not correct the issue. The easy way to fix it is to go to Stand-By (e.g. close the lid or stand-by button). If I ignore it long enough, it will go back to the normal display sometimes in a matter of several minutes and other times much longer without touching anything on the keyboard or screen. It used to happen about once every other day, although has not occurred in the past week or two. Only happens in my regular eight cell battery mode. I don't recall having the issue while using the extended battery attached to the bottom of the tablet. It has not been annoying enough to force any corrective actions on my part.
Full disclosure: Not a Lenovo employee, just someone with a X61 running Win XP SP2
