A small firmware which boots off a FAT32-formatted SD card and totally blanks the kernel, filesystem and user portions of the NAND fixing 'bad block syndrome' often caused by 3.0->2.0 downgrades (though it can happen simply by writing to the NAND often enough).
It leaves the bootloader in tact.
Full instructions in README.TXT
http://www.gp2x.de/cgi-bin/cfiles.cgi?0,0,0,0,42,2490