Kgsws, a PSP coder, has managed to find another exploit that has been tested on a PSP 1000 on firmware 5.51. This is the second version of the exploit because the first needed a PSP running CFW and you need to be connected to a wifi network.Here is a video demonstrating the exploit:
You can try it out yourselves by downloading this file: MOH Exploit v2 (40)
More information on how to install is included in the info.txt file in the download above.