Looks like its fairly easy to hack RFID tags.
Researchers at Edith Cowan University have proven Generation One radio frequency identification (RFID) tags can be breached to cause a denial-of-service (DoS) attack on the tags, using cheap store-bought radio transmitters.
Generation One tags, currently used by the U.S. Department of Defense and many Australian organizations engaging in RFID trials, operate in the 902-938MHz range. Researchers have proven a DoS attack on the actual tags will cause them to enter an error state, allowing someone to input incorrect prices or alter location and destination parameters. Full ArticleAnd this was done by Australians, I wonder what a real hacker could do.