sasuke2911 has ported Chui's MAME4ALL to the PSP. The release was on a French forum. And since I never paid attention during my French classes in school, please excuse me. I've translated his release notes using Babelfish.

He has tested the emulator with 2 ROMs. Although the menu is using the whole PSP Screen (480x272), the games will only use a resolution of 320x240. The ROMs must be put at the root of the Memory Stick. And the root isn't the MP_ROOT folder, it's the drive which you open in My Computer. So if your PSP is at F: then your root is F: itself.