Here's a link to a good article about the 1940 census by Diane Haddad of Family Tree Magazine.
http://blog.familytreemagazine.com/insider/2012/03/23/1940CensusSimplifiedWhatYouReallyNeedToKnowIn7KeyPoints.aspx
http://blog.familytreemagazine.com/insider/2012/03/23/1940CensusSimplifiedWhatYouReallyNeedToKnowIn7KeyPoints.aspx