Back around 1980 or so, I bought a book on how to make a robot pet. It would let you know when it was hungry (low batteries), follow you around, and avoid obstacles (ultrasonics). Well, microcontrollers have come a long way, and the ability to imitate life has improved greatly. Check out this example of a modern robotic “dog”:
http://feilipu.posterous.com/?sort=&search=dogbot