Our affiliates over at Neoflash who are the makers of many Nintendo DS Flashcarts and a whole host of other hardware creations have posted some info to their Mystery project.
Guesses so far are
Touchscreen
A motion sensor
Now DRNeo posted that one of them is the correct answer.
A PSP Touchscreen would open up many doors as a keyboard or the many ways the DS Touchscreen has been utilised.
A motion sensor would be excellent in a different way for the PSP.