Many analysts predicted that there would be a massive increase in turnout in this year's election. In actual fact, that doesn't seem to have occurred. With 97% of precincts reporting, CNN indicates that there were just under 120 million votes cast in the presidential election (63.4 million for Obama, and just over 56 million for McCain). That is pretty similar to the 121.4 million cast in 2004. Obviously, as the remaining precincts report in, the 2008 total will climb, perhaps reaching 125 million or somewhat more. That increase, however, will not be significantly greater than what one could expect based on population growth. The US population increased from an estimated 293 million in 2004 to approximately 305 million today. This 4% increase in population is almost exactly the same as the 4% increase in vote totals between the 2004 election and 2008 (assuming that this year's popular vote total ends up around 125-127 million votes or so). It's possible that the remaining precincts that have yet to report in are very densely populated, containing much more than their proportional 3% of the population. Still, I would be surprised if the final vote count rose significantly above 127 million or so (a 5-6% increase over the currently tabulated vote).
We also have not had the major anticipated increase in relative turnout by younger voters. Exit polls show that voters between the ages of 18 and 29 were 17% of the electorate in 2004, and 18% this year, a statistically insignificant difference. It is, however, worth noting that black turnout increased from 11% of the electorate in 2004 (almost exactly equal to the African-American percentage of the population) to 13% this year. Quite understandably, black voters were enthusiastic about supporting the first African-American candidate with a real chance to win the presidency.
I am not a big fan of increased turnout, which I think is an overrated objective. Increasing voter turnout tends to increase the percentage of voters who are likely to be highly ignorant, an already very serious problem in our politics. As a general rule, turning out to vote is correlated with education and interest in politics, which in turn are highly correlated with one's political knowledge, as I discuss in this article. An increase in turnout is likely to attract new voters with lower average knowledge levels than the previous electorate. In this 2006 post on turnout, I emphasized that there is no evidence showing that increased turnout improves the quality of government in any way. It worries me that so many people are concerned about increasing turnout and relatively few worry about the fact that most of the voters have little understanding of the issues and candidates they are voting on.
That said, I think the lower than expected turnout is a good sign for Obama and the Democrats. It suggests that their victory was not primarily driven by first-time and "sporadic" voters who are unlikely to return to the polls in future, less emotionally charged elections.
UPDATE: Some commenters note that other sources predict turnout to be around 130 million, rather than the 126-27 million I estimate based on the CNN numbers; it was probably a mistake to rely on the (still incomplete) CNN figures. However, the larger estimate of 130 million still represents only about a 6.3% increase in vote totals over the 122.3 million the same source indicates were cast in 2004. That is still only a slight net increase above the 4% population growth over the last 4 years.
UPDATE #2: In response to commenters who worry about the distinction between total population and those eligible to vote, it's worth noting that the total "voting eligible population" (US citizens old enough to vote minus those ineligible because they are convicted felons or for other reasons), increased from about 203.5 million in 2004 to 213 million this year. This is a roughly 4.7% increase, very similar to the 4% increase in the general population during the same time period. If anything, using the VEP figure rather than the general population figure makes this year's turnout increase seem even smaller.
UPDATE #3: Some experts are predicting final vote totals of about 134 to 136.6 million, which would be a much larger increase over the 122 million of 2004. That would be a roughly 10-11% increase in total vote, still significant even after factoring in the 4.7% increase in VEP noted above. Michael McDonald, the George Mason University scholar quoted in the above article as predicting the highest turnout number for this year estimates a 64.1% total turnout rate for 2008. However, it's worth noting that his calculations show a 60% rate for 2004. An increase from 60.1% to just over 64% is statistically notable, but hardly earthshattering.