Dark-AleX has just released a hack for the PSP that boots from the Memory Stick, and will allow you to select any firmware version you so choose. The app, dubbed Time Machine, will also boot-up PSPs with destroyed firmware, a second life for you not-so-great hackers out there. It functions via Pandora, and looks a bit tricky to get to grips with at the moment.
Nevertheless, the nifty bit of software will work on both the original PSP and the skinny, prettier version too. We just tried to grab the app from the link over at DCEmu, but it appears the servers are down from all those individuals trying to resurrect their PSPs like a phoenix from the flames. It's a waiting game for now, but as ever, proceed with caution. [DCEmu]