One of the reasons I love ETech is talks like this one from Chris Anderson (of Wired) on building homebrew drones, or unmanned aerial vehicles (UAVs). He has a Web site that shows how to build the various kinds of drones he talked about. He's used Lego Mindstorms, cell phones, and microcontrollers on planes. The results are pretty astounding.
He wanted something you could do indoors, and hit on the idea of using blimps--which are inherently autonomous since they float. The blimp uses ultrasonic sensors to maintain altitude. When it's powered up it looks determines it's altitude and then holds that. He has infrared beacons that serve as way points. The blimps don't have an absolute frame of reference, but know where they are in the room. Chris describes it as having "sub-Rumba-level intelligence."
Chris has a "blimp board" that he is using for the next generation blimp with motor controllers, compass, ultrasonics, and a even a way to know room temperature (which makes a huge difference with a blimp).