Update: There is already a new version released of the GameBoy Color Emulator detailed below! Improvements include it running at Full-Speed with no noticable graphic issues now, however, it still has no sound support as of yet. There is also a ROM selector v0.01, File Selector v0.02, and File Selector v0.03 released for use with it now too!
Today a GameBoy Color Emulator has been released for the PlayStation Portable by Mr. MIRAKICHI! Currently, it only works on PSP units with firmware v1.00 installed, and doesn't support sound as of yet. In addition, you must rename a ROM to ROM.GB and put it to the same folder as the included EBOOT.PBP file. Finally, http://psp.ign.com/articles/611/611529p1.html?fromint=1 also has it the Japanese PSP is going to be getting v1.51 firmware soon, so keep checking those buffer overflows from the v1.00 to aid in getting v1.50+ units caught up. Feel free to discuss the cool news in THIS Forum thread. Enjoy!