The frequency of the bug’s occurrence is definitely tied to system / graphical load. With nvidia cards a few firefox windows (or anything with heavy xrender use) and/or a few open windowed opengl apps and the bug happens about 50% of the time when changing screens and happens much more frequently when moving over the boundary between GPU’s than it does moving over the boundary between x sessions that are running on the same GPU. To “reset” the bug without rebooting the x server you can do the following:
alt-tab to a window (firefox is the one i usually use)
alt-space to bring up the window options menu (metacity)
select move (this binds the cursor to the center of the window)
use the arrow keys to move the window to another X session window
move the mouse the bug should now be gone
V6