There are now
Nintendo Entertainment System and
SNES emulators for the PlayStation Portable. Of course, they will only run on Japanese PSPs with the old 1.0 firmware, which doesn't restrict the running of code from the Memory Stick, and that's only until someone runs a game which automatically patches the firmware on the unit.
Let's hope that they figure out a way of getting homebrew code to run on a PSP, as that looks like a pretty nifty platform to run things on.