Not enough is known about the PSP to emulate it. If the information were out there, you can guarantee that a PC emulator would exist! That said, it is extremely impressive that the PS3 can emulate the PS2's EE and it is expected to be able to emulate both the EE and the GS in the future. That said, the PSP will actually be a little harder to emulate, even though it has the perception of being less powerful than the PS2.
My point is that it's silly to ask these questions if a PC emulator for the same platform does not exist. The only time console's have emulation that PCs don't is when the information used is private to the emulator developer (Sony built the PS2 and owns internal PS2 documentation (not public) that makes it possible to emulate the PS2 on the PS3 and they also have the budget to pay for such an enormous effort. Similarly, they have all the information about the PSP. Until it is reverse engineered and/or leaked and put to use on the PC, forget a PS3 version (that isn't from Sony).
I would not say that it's something Sony would never do. Think about it: Digital distribution of PSP games over the PSN store to connected PSP and also sellable to PS3 owners as downloadable content. To developers, their game is dual purpose (PSN download for PS3 owners, PSP download for circumventing the stores/selling Memory Sticks).