Just reading over this thread got me thinking, you want to turn the computer on when the power switch is flipped, off when its flipped off, but it would be advisable to also have the computer instead of just powering straight off (for those running windows) to start shutting down instead.
about $10 in components should do it. a 555 timer, a couple caps, resistors, and a relay. Basically, you need a rising & falling edge trigger, with a delayed pulse one shot.
you'd plug the computer into constant power, and the "device" into the switched mains, and a small cable from the "device" to the power on header on the MB.
When you turn on the cabinet, the relay picks up, triggering the "device", after a second, it would pulse the power on header for a second, starting the computer.
Same thing on power off of the cab, the drop of the relay would trigger the "device", pulsing the power on header, initiating a shutdown, or hibernation, or whatever you've got it programmed to do. No hard crash of the computer.
I agree that the way Peale has it set up must work, and if your not concerned about crashing the machine, its a easy way to go.
I've seen too many times though a machine just getting its plug pulled killing the HD, or the CPU, as it doesn't get a chance to shut down gracefully. When you just pull the plug on the PSU, the 12v and 5v lines surge momentarily, and can kill components (HD's usually die first).
If anyones interested, I'll draw it up quick. Everything for it would be availiable at radio shack, and for probably less than $10. You'd need to be able to solder though.
SD