Update #4: Today Miemt11 has just released GBA Emulator v0.01 For PSP. This is his first GBA Emulator attempt, and is based on the psp298 GBA v1.1 source code. Features currently include a smaller EBOOT file size and also (untested) PSP v1.00 Firmware support for those interested.
Update #3:PSPGBA v1.1 is now available, and changes include a NiCE speed increase, the addition of 240x160 screen mode, and the implementation of vsync on/off which also aids in speeding up some titles.
Update #2:psp298 has released a new GBA (Game Boy Advance) emulator for the PSP, namely PSPGBA v1.0. Currently it includes a GUI for game management (like RIN) and is optimized for speed, however, it's currently not 100% as fast as GBA but many games are definitely playable so enjoy!
Update:VBA for PSP v0.03update has now been released, and although the ReadMe states there is no sound implementation yet it appears there is (pushing Left/Right on the Analogue Stick seems to toggle the sound ON and OFF). Changes also include supporting one game at a time (it must be renamed to ROM.gba), the directory must be named VBAPSP, and one SAVE at a time total per game is now featured. However, if you wish to save on another game you must remove the old save to do so. Similarly, pushing Up/Down on the Analogue Stick may change the clock speed... as it changes the background from Black (222mhz), to Blue (266mhz?), to Red (333mhz?) and provides a noticeable speed difference on the Red background. Lastly, pressing Triangle/Square increase and decrease frame skipping it appears... some nice 'undocumented' features indeed!
Today a GBA emulator (Visual Boy Advance port) for the PSP has been released! Currently it plays very slowly and has no sound, but the Japanese author states improved versions are coming soon. Also, it appears that ROMs larger than 16MB arent working yet. Those interested can grab VBA for PSP v0.01 to check out for yourself along with the ROM File Selector, and to use it simply extract to PSP/GAME/VBAPSP and rename your ROMs to ROM.gba. Enjoy!