What would be the point? The MDD/FW800 topped out at 1.42GHz, the current top G4 upgrade card is 1.5GHz. There isn't really any point in upgrading a CPU unless you're close to doubling the clock speed so the market isn't there yet.
Wait until Freescale have their 2GHz dual core e600 out or something similar then I think you see viable upgrade options for the MDD/FW800 machines.