Not sure who was really first
with this, but it seems like this bit of
PSP lore is emerging simultaneously from multiple sources
as folks discover it. There's a fairly simple way to achieve multi-player gaming on two PSPs using a single UMD disc,
by starting the game on one console, starting an Ad Hoc Multiplayer game, then hot swapping the disc to the other
device. It's apparently working for Tony Hawk, Twisted Metal, and Ridge Racer with perhaps a few protocol tweaks here
and there. More complete instructions via the Read link.
[Thanks to everyone who sent this in]