As a victim of this prank (in the late 90's) I can attest to a) how puzzled you feel when nothing responded to mouse clicks and b) how funny it is when someone finally reveals what was going. The sole other programmer I was working with at the time had set me up, and managed to keep a straight face for 30 minutes while he suggested various solutions (rebooting, a different mouse etc).
I did something similar to this but made the desktop upside down. Then inverted the mouse so that if someone pulled the mouse, the cursor would go up (as if the screen is upside down).