The U.S Voter Participation: 1990-2010 infographic examines the data trends of the last five Presidential elections and by-elections. The data would suggest that the number of people coming out to cast their votes in American elections is definitely on an upward trajectory.
In the 2008 election (in which Barack Obama dominated), people were motivated -- young and old -- to go out and support their candidate, because it mattered. As the Republican and Democratic national parties become more and more polarized, voters are finding that their individual vote matters more and makes more of a difference. Gone are the days when one can be apathetic about politics whilst living in America, because so much can change (for better or worse) depending on your individual cast ballot.