The 1940 US Census record will be available to us on 4 April 2012, but you can get ready for it now.
When it is first released, there will be no indexed searches available. That means to use it you have to be aware of where you ancestors lived in 1940 in order to find them in the census.
More and more articles are being written about the census release, and a helpful one comes from Randy Seaver in his Blog "Genea-Musings." You can read Randy's article at http://goo.gl/ee53l