Note - this circuit requires a receiver with a 12V trigger output or some way to trigger a 12V signal from an IR remote.
I have a Pronto universal remote set up to control everything in my Home Theater including powering OFF my Boxee computer -- but the only way to turn on the computer was to walk up to the equipment and push the power switch (gross!) So I designed a small circuit that powers on the computer from a +12V trigger. I have a Pioneer SC-05 receiver which has two assignable +12V trigger output and I set it up to turn on Trigger #1 when I switch to the Computer video/audio input. The circuit is totally passive (Relay logic! <g>) and uses two relays you can buy at Radio Shack. Here is the circuit diagram and a picture of the competed switch (sharp eyes will notice the diode is missing - this is Rev 1.2, I added the diode in Rev 1.3). (Sigh this forum's limit on pdf's is so small I can only attach a crappy version of the schematic.)
The way it works is:
The relays are connected in parallel with the normal computer power switch (which therefore continues to work normally). When the 12V trigger comes on, the power switch header pins are shorted through the relay contacts (just as the manual power switch does). Once the computer powers up, the 12V fan header actuates relay 2 which opens the circuit. As long as the computer is on, relay two is actuated and the circuit does nothing (I use IR to turn the computer off). Therefore it is ok to switch the 12V trigger on and off while the computer is powered on (by switching to another input for example) - nothing happens. Pressing the manual power switch while the computer is on powers it off normally.
The 1K resistor is only there because I originally used it as an inductive snubber, but then got paranoid about allowing any negative voltage back into the SC-05 12V trigger output (although I hope Pioneer protects their outputs with snubbing diodes) and so I added the diode. The only downside of the diode is the input circuit becomes polarity sensitive - you have to make sure you get the cathode (banded) end of the diode connected to the tip connector of the jack. If you use the diode you can leave out the resistor. If you use the resistor you need to pick a relay that draws no more than 38 mA to avoid exceeding the 50 mA trigger output. You could also use a 5V relay and a dropping resistor chosen to drop 7V at the relay current.
The whole thing is built on a 1.5 inch square of Radio Shack perf board and just sits in a empty spot in the computer. The jack I chose fits perfectly in an unused round audio cutout in the back panel. You will also need a three foot monoplug-to-monoplug cable to hook up the computer to the receiver. Don't forget to go into the receiver setup and turn on the 12V trigger for the Computer's video/audio input.
If you wanted to have your Boxee computer always come on when you power on your Home Theatre, you could switch to the Boxee input in your power on macro.