FPGA emulations of many CPU's has already been done - even relatively low end FPGA's are doing this, take the Papillio.cc as a very low cost example. Jack Gassett is behind this project and its an easy starter for people who want to dabble in FPGA's with minimal outlay.
There are a list of freely available cores Here. Open the Processors tab and marvel at the list. 8080, Z80, 6502, 8051, ARM, 68000, GPU, MSP430, PDP 11/70
People have even done complete computers such as the VIC20
Nintendo NES Nintendo NES
Many of the higher end FPGA platforms come with standard CPU "Soft Cores", google for "soft core FPGA" for more info. Cores such as LEON3, MicroBlaze, Nios II, OpenRISC are available.
FPGA has a place as does a CPU. Its a bit irrelevant where it actually lives - ie CPU within FPGA, CPU with FPGA capabilities etc.