The US Census for 1940 was released on April 2, 2012. There are 4 or 5 sites that made the images available within the first week, but all did not have the names of people indexed. Over the past 3 weeks or so a few states have been released with an index by different sites.
Without a complete index, the work to find selected individuals is much more difficult than having an index available.
But it can be done. Check out www.g4beginners.blogspot.com//2012/04/how-to-find-relatives-in-1940-census.html to find out how.