This stirred a deeply (well, not that deeply) buried memory in the hive mind that is Engadget, and we hunted down a Sony ad that's run on Japanese TV recently, which shows popular magician Tomohiro Maeda messing with a couple of PSPs and demonstrating playing a networked game on them with a UMD inserted into only one.

ENGADGET: Multiplayer PSP games with one UMD to be a feature, not a hack?