Every state has finished compiling the votes cast for president, for the candidates who were on the ballot. Some states are still working on their write-in tallies. In six states, fewer votes were cast for president in 2008 than in 2004. They are Maine, Oklahoma, Oregon, South Dakota, West Virginia, and Wisconsin.
Overall, according to Curtis Gans, turnout in the U.S. was 63.0%. Gans calculates that 208,323,000 people could have voted if they had wished, although of course that would have meant that they needed to register. Gans calculates that 131,257,542 votes were cast for president, which yields the 63.0% figure. Gans calculates 60.6% for 2004, and 54.2% for 2000. This suggests that 2008 did not have a huge improvement in turnout. The 2004 gain in turnout (compared to 2000) was more dramatic than the 2008 turnout gain.
Curtis Gans has been working on turnout data for many decades, and is associated with American University.