Just came across the following item on maplin:
Three units and a remote that go between an appliance and a wallsocket, allowing you to wirelessly turn off an item at the mains. Works for 25m and through walls, so it must be rf or ultrasonic. Looks like it could make part of a nice hack. And the best thing is that it costs £8!
Anyone played around with anything like this before? I might order one in..