My MAME cabinet uses a Mac Mini, however the Mac has an internal power supply so unlike a PC it's tricky to wire up a remote switch without pulling the Mac apart. So instead of dismantling my Mac I bought a small 5V solenoid and mounted in so that the plunger depresses the power switch on the Mac - easy! The solenoid is hooked up to a momentary switch mounted on the outside of the cabinet.

This is the solenoid: sparkfun.com/products/11015

This thing prints on its side. The 2 long slots are to allow the solenoid to be moved as required to get the right pressure on the power switch. The long flat piece is to align the mounting plate with the side of the Mac.