Interesting move from Google. I first read about this in Tech Crunch. I just saw one of these billboards in San Francisco, right on the 101 by the 4th/Harrison on-ramp.
Makes sense to me. Google is flush with cash and offline media (billboards, newspapers, etc) is the best way of driving mobile voice/text traffic since a person is not in front of a computer and needs the info right away! This Google service competes with Free411, which happens to be a 4INFO partner.