I believe that there are some FPGAs with embedded PowerPC cores, so you don't need to re-implement them in VHDL. I don't know the price, nor the performance of the cores.
I'd like to know more about that (the above statement).
And to echo what's been said, an FPGA implementation of a PPC core wouldn't be practical as it would be too slow. There is nothing to prevent a PPC add on card for the Natami from being designed though.