now that might be a fun robot...
it reminds me of: Polar bears destroy robot spy cams, show no remorse (video) | Engadget
wonder how long it would take the cat to do the same. but yeah, a cat sensing robot might be an awesome toy to build.
still, simplest robot I can think of has two sensors, or one sensor mounted on a servo, and a differential drive using two continuous servos. Add an arduino for upwards compatibility to later stalk cat or mailman.