Ancestry.com is adding the 1940 US Census images to their website. Here are some things to know about searching them.
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. Once we receive the census, we will begin uploading census images to our site so the public can browse them. Initially, this collection will be what we call 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. At the same time, we will be working behind the scenes 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. Note also that the 1940 U.S. Federal Census will be accessible free of charge throughout 2012 on Ancestry.com.
As of this morning they have some images uploaded but it will take time. Remember they have 3.8 million images to upload.
Ancestry also has enumeration maps. Enumeration district maps outline the area included in each district, and they can help you tremendously when you want to determine which district included your family’s residence at census time. You’ll find the maps at http://search.ancestry.com/search/db.aspx?dbid=3028.