Daily Kos pulls a piece of data from its last Research 2000 poll that suggests Democrats will be less than half as likely to vote in the 2010 midterms as Republicans and independents:
QUESTION: In the 2010 Congressional elections will you definitely vote, probably vote, not likely vote, or definitely will not vote?The results were, to put it mildly, shocking:
Voter Intensity: Definitely + Probably Voting/Not Likely + Not Voting
Republican Voters: 81/14
Independent Voters: 65/23
DEMOCRATIC VOTERS: 56/40
A few things to keep in mind: first, as Jim Geraghty points out, it's a long time between now and 2010. It's also possible that the "Democratic" category is a bit inflated, at least compared to who you'd think would vote in 2010 anyway. The Republican/Independent/Democratic categories only include respondents deemed "likely voters," and, while Research 2000 doesn't release the questions it uses to determine that, it seems that voting in 2008 is probably one of the qualifications.
So some marginal voters who don't usually vote, and may have little interest in voting in a non-presidential race but went to the polls in 2008 to support Obama, might be getting counted in that category.
Research 2000 also hasn't asked this question before, so there's no consistent set of data to compare this to.
That said, Democratic turnout is expected to subside in 2010, after the massive turnout Democrats saw in 2008. The Obama voters, it is assumed, won't turn out in droves like they did last year. As the Democratic tent has gotten bigger in the past year, the question is how significant the drop-off will be.
Research 2000 says it plans to keep asking this question, with the language tweaked just a bit, so this will be something to keep an eye on as 2010 approaches.