This is one of the Arduino projects I am going to try out - it goes very well together with out already RFID enabled catflap (see elsewhere on this blog for more):

Some people love to keep cats as their pet. However, unlike the dogs, cats didn’t like to stay at home and its like to wandering all around the neighborhood. Sometimes, you’re very hard to track its trace, as it can walk up to several kilometers away per day!

If you’re one of the cat owners and would like to keep your eyes on its, maybe this CATaLOG project is what you’ve looking for…

The CATaLOG has the ability to track your cat as they go in and out of the house. By using this intelligent RFID cat tracker, you can easily find out:

The times your cat spends outside of the house.
The times your cat mostly gets out from the house.

[From RFID cat tracker | Embedded projects from around the web]