On March 7, the Bipartisan Policy Center released this study of turnout in 2012 presidential primaries. It shows generally lower turnout than in past presidential primary seasons, but notes that 2012 presidential primary has been comparatively better in the states with open primaries or semi-closed primaries.
So far, the worst turnout in 2012 has been in Virginia, where only 4.6% of the eligible electorate chose to vote in the presidential primary.
the Bipartisan Policy Center was formed in 2007 by former U.S. Senate leaders Howard Baker, Bob Dole, George Mitchell,, and Tom Daschle. Thanks to Rick Hasen for the link.