See this press release about this report issued by the EAC. Among the conclusions of the report:

– Nearly 173 million persons were registered to vote for the 2006 elections–an increase of nearly 12.1 million since the 2002 midterm election.
— The number of registered voters declined since the 2004 Presidential election from nearly 176.2 million to 172.8 million.
–76.6 percent of the Voting Age Population (VAP) was registered and 83.8 percent of the Citizen Voting Age Population (CVAP) was registered.
–More than 82 million ballots were cast or counted in the 2006 election.
–One percent of those participating cast a provisional ballot. (slightly more than 794,000 individuals)

This should make for interesting reading.
UPDATE: See also Ohio provisional ballots: Could they determine our next President? by Nathan Cemenska of Moritz which uses the election day survey data to talk about this issue.