A great paper was published about the possibilities of spreading viruses through RFID tags.
The original paper can be found here (http://www.rfidvirus.org/index.html).
A summary can be found here (http://www.securityfocus.com/brief/163).
That doesn't matter, though; the ID numbers that are stored in the RFID only lead to a database entry, and perhaps a phone number, address, and name. What's so dangerous about that?
I don't suppose you read the article or paper?
If the software that reads the RFID tag is vulnerable to an overflow, then an overflow can be programmed into your cat's collar. When the cat walks into your house, it can plant a virus on your home network.