There was a brief mention of this technology at the Crypto conference.
I provided some pointers in a comment to an Ed Felten blog entry at
http://www.freedom-to-tinker.com/archives/000677.html#comments (scroll
down to the 3rd comment).
Dan Boneh et al presented a proposal for a group signature
http://www.boingboing.net/2004/10/01/federal_program_to_m.html
Federal program to monitor everyone on the road
Interesting article about the Fed's plans to develop an all-knowing
intelligent highway system.
Most people have probably never heard of the agency, called the
Intelligent