Guide to International Migration Statistics: The Sources, Collection, and Processing of Foreign-born Population Data at the U.S. Census Bureau [open pdf - 279KB]
"This report presents an overview of information available from the U.S. Census Bureau about the U.S. foreign-born population. Information about the foreign born is available from a variety of sources including: decennial censuses, current surveys, and demographic estimates based on administrative sources. Specifically, this report describes surveys fielded by the Bureau, the international migration-related items covered by the surveys, and the post-collection processing of these data."
Census Bureau: http://www.census.gov/