The Null Device
This guy attached a digital camera (a specially modified keychain camera) to his cat's collar and recorded the sequences of images as the cat (named Mr. Lee) went about his daily journeys. The images vary in quality (as one might expect), and include a lot of obscure nooks and crannies of the sorts cats frequent, and quite a few meetings with other neighbourhood cats.
The author of the page, J. Perthold, also sells cat-mountable digital cameras of his design for people who want to photograph what their cats are seeing. The next step, I imagine, would be to integrate wireless networking capabilities of some sort into the unit and set up a catlog.