As of this morning, April 2, 2012, the 1940 U.S. Census is available for public use.
For privacy reasons, each census does not become available to the public for 72 years after it is taken.
Check out this National Archives web site to learn how to use the 1940 Census, which can be tricky because you can't search by name. You need to search by location, and thus must have a general idea of where the person lived that you are looking for. But it is an amazing resource for all you budding genealogists. Happy hunting!
For more specifics on the census and Brooklyn, check out this online exhibit at the Brooklyn Historical Society about what we can learn from the census about trends in interracial marriages in our borough.