if you could open up the computer, you could also build a relay to the power switch that had its own power supply that is turned on either by an input from an IR led, or by a network somehow. im still looking around for how to do that.
*EDIT*
ok, as for the network turning it on, you could just put the computer into hibernate mode and get a wake on lan card. supposedly, these cards can take a pc out of hibernate mode if a packet is recieved telling the computer to wake up.