Although unanimously endorsed by public health and medical organizations, federal bills aimed at phasing out the non-therapeutic use of antibiotics in US food animal production have repeatedly failed to make it out of committee. The current version is still in committee - the 'Preservation of Antibiotics for Medical Treatment Act of 2009'.
Govtrack.us is a pretty neat site for tracking legislation. If you click on 'related legislation' you can see that legislation with the same title was introduced to the previous 3 Congress's but has never made it out of committee despite having bipartisan support (at least the sponsors include both Republicans and Democrats).
You can read the FDA's Statement before the House Committee on Rules on July 13th this year, here. A statement read into Congressional record by Rep. Jerry Moran (The expanding power of the federal government and its intrusion into America's business) gives some idea of why this Bill has trouble getting anywhere.
Other countries have, however, acted, the European Union banned the use of antibiotics as growth promotional agents in 2003.