I'm using a laptop, a Dell one. I bought a cheap docking station for it, £20 at a computer market. I have wired two internal relays to it, it now powers on everytime the poewr is connected, and once power has been removed, it goes into hibernation after 10 mins..
the relay diagram I used is
It works great :-) and also means I didnt open the laptop, and dont need to disconnect any wires if I want to bring it inside to work on it.