Chris Myers over at isopod design created a home defending system by using a Roomba. He salvaged a red Roomba off Craig's List and gutted out the vacuum. Using Make Magazine's bluetooh tutorial and a bluesmurf bluetooth module from Sparkfun, Chris managed to get the Roomba wirelessly. Finally he added in an Airsoft gun and lazer to make it complete.
Here is a YouTube video of the Roomba in action.