It only happens once a decade. Seventy-two years after the 1940 US Census was recorded, it is being released to the public. Unlike previous years where it was released on microfilm and took a long time to digitize and then index, this time the census is being released fully digitized. You should be able to see census pages on day 1 (April 2), although no index exists so you will not be able to search by name or address. The work on such an index will only start on April 2, 2012 when the files become available.
There are at least two efforts to index the 1940 Census records, one by Ancestry.com, and another by a consortium called the 1940 U.S. Census Community Project, which includes FamilySearch.org, Archive.com and FindMyPast.com.
It is unlikely that a full index will be completed by either group before at least six months from now, and probably longer, but there is a way to find the enumeration district (ED) for your relative which is the neighborhood that the census enumerator recorded.
Stephen Morse and Joel Weintraub have collaborated to create a number of useful tools that help you figure out which enumeration district you should be looking for, and when the images are released you will be able to locate which images are from the enumeration district you are looking for. To get started with these tools, use the quiz page on Stephen Morse’s site. If your relatives were already living in the US in 1930 and didn’t move by 1940, you should try to find their census records from 1930 which will help in figuring out the correct ED in 1940.
Be sure to go to Stephen Morse’s site before April 2, as it will likely be difficult to reach on April 2 and for some time afterwards. Find the EDs of all your relatives who lived in the US in 1940 now, and then on April 2 you will be able to go directly to the census images and find your relatives using the EDs you already found.