Chennai will no longer process Immigrant Visa petitions as of January 1, 2012.

"Important Announcement: The U.S. Consulate General in Chennai will no longer process Immigrant Visa petitions as of January 1, 2012. The U.S. Consulate General in Chennai will no longer process Immigrant Visa petitions as of January 1, 2012. The U.S. Embassy in New Delhi and U.S. Consulate in Mumbai will become the only acceptance centers in India for Immigrant Visas.
The Immigrant Visa allows an individual to live and work in the United States on a permanent basis, as well as provides the immigrant the opportunity to work towards U.S. citizenship.
Examples of Immigrant Visa based petitions are those that work toward family reunification of parents, children and siblings. These petitions also include Fiancée and the unique employer category, where one’s employer can petition for the individual and their dependents to immigrate to the United States. This change affects only the locations for processing of Immigrant Visas; it does not change the law or policy for visa processing. Applicants currently in the process of petitioning for an Immigrant Visa may contact ChennaiIVU@state.gov for clarification."

Source: The U.S. Consulate General in Chennai