It is released on April 1st. Of course, we can never be certain that this isn't the April Fools' Census, so I'd wait until April 2nd before using any of the numbers.
April 1, 2010 or April 1, 2011? The reason I ask is April 1, 2010 is just 3 months into 2010 and that wouldn't be much time to distribute the census, fill out the census forms, collect the filled-out forms, and tabulate the results, unless they are either really quick about things, or they start distributing the forms in late 2009.
Plus, the 2000 Census, if it was released on April 1, 2000 wouldn't take effect until January 2001 since the 2000 Presidential Election were the same EV's as 1992 and 1996.