My friend has a media pc with a remote. He can press the power button on his remote to turn his computer on and off.
If you have an understanding of electronics you could accomplish a similar effecft. You just need an infrared emitter (remote control would work fine), and a receiver built into your computer. You could rather easily construct an electrical device so that when the receiver receives the signal it turns the computer on/off. If your interested in doing this mod yourself and would like some help with it, shoot me a PM.