That's odd. We thought they squashed that bug a long time ago. According to Fanjita, Sony didn't quite bring their A-game when it came to stopping their unruly fan base.
It's not all sunshine and unicorns for those itching to tear apart the coding that holds their PSP together. The brave will need an original copy of Grand Theft Auto: Liberty City Stories (later copies were updated to prevent this trick from resurfacing) and nerves of steel. An "easy mode" downgrader isn't available yet, but the wait should give you enough time to scrounge up a stack of PS One games to stick in there.
[Thanks to everyone who sent this in.]