The bad news:
The 1940 United States census will not be released until April 1, 2012.
The good news:
If you have completed your 1930 census work, and recorded the ED (Enumeration District), you'll be able to find the new ED in the 1940 census. This will be useful to those who do not want to wait until a name index has been created.
I used one of my families, and entered the Louisiana parish name, ED60, and the suffix, 14, into the appropriate places on the conversion page. It told me that ED60-14 will be partially or fully contained in EDs 60-14, and 60-15A-B in 1940. So, when I do get a chance to look at the 1940 census, I'll be able to jump right to the correct ED and begin searching for my family.
Incidentally, 1940 will be the first census in which many of us will be able to find ourselves with our parents, or guardians.
You might not need this now, but store the website in your browser favorites. You'll be prepared when the 1940 census hits the libraries and on-line websites.