"Personal firewall for the RFIDs you carry"

Aaron Burstein recommended this paper by

Melanie R. Rieback, Georgi N. Gaydadjiev, Bruno Crispo, Rutger F.H.
Hofman & Andrew S. Tanenbaum, "A Platform for RFID Security and Privacy Administration," which discusses "a device that sits on your person and jams the signals from all your personal wireless tags (transit passes, etc), then selectively impersonates them according to rules you set." See also the Boing Boing post