Hmmm.... I'm not a huge fan of RFID tags given some of the comments made about RFID technology at Defcon - particularly about reading chips from say 30 feet away.
Ars have an article about tags being used to store and transmit malicious code
- for a useful technology, it looks badly implemented.
http://arstechnica.com/news.ars/post/20060315-6386.html