So, you have a friend with Vista, or XP planning to upgrade to Vista, or a Mac person upgrading to Leopard. Well, here is your chance for the perfect storm of pranks.
Vista SP1 was just released today (downloading it now - had to refresh Windows Update a few times to get it to appear, but was 65MB, vs 450 for the standalone installer). Any of these semi-major upgrades of computers could always have "unresolved issues" deflecting the troubleshooting-est of troubleshooters towards software problems.
Sooooo... if you were to get one of these random keystroke generating USB ditties, they would probably go insane before figuring out the problem.