The PSP hackers over at noobz.eu have released a killer tool for fixing any bricked PSP, restoring it to v1.5 firmware. When the PSP boots, the hardware checks for the serial number on the battery eeprom. If it’s 0xFFFFFF, it loads firmware from the memory stick instead of from internal flash. You use a working PSP along with the Noobz team’s tool to create one of these special batteries. With it, operation can be restored on devices that were previously unbootable due to corrupted firmware. It’s also a nice way to completely downgrade your PSP. Pretty cool stuff.
Pandora’s Battery: unbricker, downgrader for all PSP firmware versions – Link