This is a really handy tip for wanting to keep your computer awake or keep grooveshark playing, using a cup.
Laser mice only...
I don't use this hack myself, my desk is glass and causes this effect anyway, plus the mouse wets itself if you don't use a mouse mat like the ever classy sheet of a4 paper.
I broke a wineglass the other day, one of the pieces suits this purpose perfectly, though any lump of transparent material will do.