I heard about this on NPR a few days ago and decided to look it up. Turns out that Mini Cooper owners can now sign up to have the billboards lining their daily commutes customized by personal RIFD "keyfobs". The pilot advertising campaign is now running in four cities (San Fran, New York, Chicago and Miami), with plans to expand pending the program's success. I found this explanation on http://yro.slashdot.org/article.pl?sid=07/01/11/2021244&from=rss:
Interestingly, a contributer to the slashdot blog posted a comment about how it would be fun to put up fake terror warnings.
As much as I disagree about the "fun factor" of such a prank, I have to admit that he brings up a good point: there are a million ways these billboards could be misused.
Then again, giving the average consumer the means of production on such a platform is novel and potentially very liberating, with regards to constructive social and political discourse...assuming the "editors" at Mini USA permit the medium to fulfill this potential.