ToadKing unveiled what he’s been working on lately: a libretro port of PPSSPP! Meanwhile, aliaspider and Squarepusher have been working on porting RetroArch to Sony’s PSP. Put these two works-in-progress together and you have the first instance of RetroArch-ception.
Hopefully, one or both of these projects will be ready for the upcoming 1.1 release (formerly planned to be called 22.214.171.124, but there are enough major changes even without these to warrant a more significant version bump). Even if they don’t make it, though, savvy users can always checkout and build the code from their respective repositories on github.