The U.S. Census Bureau released “Voting and Registration in the Election of November 2008” on July 20. The Census Bureau’s press release is here. The press release contains a link to the report itself, and the previous reports. The Census study depends on survey data, not election returns or voter registration data.
In 2008, the Census reports that 206,072,000 people could have registered and voted. However, of those, only 146,311,000 were registered. And of those who were registered, only 131,144,000 voted. So, 89.6% of the registered voters cast a ballot. But because only 71.0% were registered, the final turnout (voters divided by the number of people who could have registered and voted) was only 63.6%.
By contrast, in 2004, there were 197,005,000 people who could have registered and voted. 142,070,000 were registered. 125,736,000 voted. So, the November 2004 turnout was 63.8%, slightly higher than the 2008 turnout.