Akira's right. Writing a JIT-PPC emulator is a *major* task - the PPC is quite hard to emulate (especially on a x86 CPU). It has 32 registers (while the x86 has only 5 of them IIRC) and some features that cause quite some overhead when you're trying to emulate them. Additionally, you would also have to emulate the MMU (Memory Management Unit), as every PPC-OS I can think of needs an MMU.
There are PPC emulators out there (I think the GNU Debugging suite includes one), but these are far from being useable (let alone "fast").
IMHO, this will only happen if a company is willing to put a lot of money behind such a project and distribute it commercially. However, I don't see a market for such a solution, that's why it won't happen anytime soon.