From Ancestry.com
The National Archives and Records Administration will open the 1940 U.S. Federal Census on April 2, 2012—the first time this collection will be made available to the public. Ancestry.com will begin uploading census images to so the public can browse them. Initially, this collection will be a browse-only collection. This means a person can scroll through the pages of the census districts much like you would look at a microfilm or a book. Ancestry.com will be working to create an index of the census that will eventually allow people to search for their family members by name as they currently can with all other censuses on Ancestry.com.
The 1940 U.S. Federal Census will be accessible free of charge throughout 2012 on Ancestry.com.