The Foundation has just published this blog post which has Amy Mather controlling electrical sockets via a Raspberry Pi. She’s using an Energenie GPIO plugin board and extension block and controlling it via Python on the Pi. You can get hold of a starter pack of this system from Energenie for £19.99 (including VAT and delivery) which will give you the GPIO board and two socket adapters. Take a look here.
I’ve not tried one of these myself but they look like a great way to get into home automation without a load of re-wiring.