Earlier this month the City Council of Bangor, Maine approved an ordinance that revised the Code of the City of Bangor to prohibit smoking in motor vehicles when there is a person younger than 18 years old present. This ordinance was effective as of January 19. As reported by the Bangor Daily News, an anonymous donor contributed $2,000 to pay for a radio campaign to inform the local community about the new measure.
Bangor appears to be the first U.S. city to enact such a measure, although similar bans have approved in Arkansas, Louisiana, and Puetro Rico. Other states are considering similar measures.
For some background information on the Bangor ordinance you can read item #3 from these minutes of the Bangor City Council Government Operations Committee (.doc format). Please note that although these minutes indicate that a violation of the measure would be a "secondary offense", in the final approved ordinance this was revised to be a "primary offense," which means that police can stop a vehicle if a violation is observed. The Bangor Daily News also has an article on the approval of this measure.