Arguru and PSP 3D have just released a new GameBoy/GameBoy Color homebrew emulator for PSP. Dubbed SpiceBoy, the emulator offers full speed FPS emulation of all GB/GBC ROMs. Though parts of the emulator were taken from InfoBOY, a Win32 GB/GBC emulator, most of the project has been handcoded/tweaked from the original project, and offers emulation as well as Homer's RIN (considering it's the first release, R0 (Release 0), coded in under a day).

A few things to note about SpiceBoy R0 are:
- There's no browser (user manually would have to replace: rom.gb).
- Sound is not synced with emulation properly.
- No savestates.

Other than that, SpiceBoy is a full-fledge, full speed GB/GBC emulator, and rest assured that a new version will be release shortly with a ROM browser, correctly synced sound, and possibly, savestates and multiplayer. Consider this a pre-mature, but working, release; it's a heck of a great job for one day's worth of work.
Download it here from Psp3d