It sounds sweet enough.  And shouldn't be to hard.  All the mobo does is sense when the power switch closes.  All you need to do is get a voice chip to send a high (or closed) signal to the motherboard over the two wires from the power switch.  The only downfall would be yelling "f*ckin' retard" in the middle of a quake game and the computer hearing system restart and your computer reboots.