The U.S. Census counts every resident in the United States, and is required by the Constitution to take place every 10 years.
The 1940 Census will be avaialble April 2, 2012!
2010 Census Participation Rates
Basic Searching: http://youtu.be/4Jt3-1dATdk (5 minutes)
Advanced searching: http://youtu.be/V77TkN0YNGQ (9 minutes)