It looks like 2012 turnout for voters 18-29 will mirror that of 2000 rather than 2004 or 2008.

Gallup:

Fifty-eight percent of U.S. registered voters aged 18 to 29 say they will "definitely vote" this fall, well below the current national average of 78% and far below 18- to 29-year-olds' voting intentions in the fall of 2004 and 2008. The 20-percentage-point deficit for young voters versus the national average compares unfavorably with six- and seven-point deficits in the later stages of the 2004 and 2008 elections, respectively.

These results are based on an analysis of May 1-July 10 Gallup Daily tracking interviews with more than 30,000 registered voters, and more than 2,800 18- to 29-year-old registered voters. In addition to asking presidential vote preferences, Gallup asks registered voters to rate their chances of voting on a 10-point scale, with "10" indicating they will "definitely vote." This analysis reports the percentage of voters who say they will definitely vote. The question is asked as part of Gallup's larger likely voter scale that will be used in the fall.

Turnout intentions are currently lower among all registered voters than they were in the month before the last two elections, with 78% saying they will definitely vote, compared with figures of at least 85% in October/November 2004 and 2008. This partly reflects the normal pattern in which fewer voters say they will definitely vote in the late spring and early summer months than in the fall of an election year. However, a comparison of similarly timed data in the 2004 and 2008 elections still suggests turnout levels this year may not match those from the last two elections. In June 2004 (80%) and June 2008 (82%), slightly more registered voters said they would definitely vote than the 78% who do so now.

Young voters were one of the key groups in President Obama's winning 2008 coalition. They widely support the president this year as well, but historically their turnout levels usually lag behind those of other groups. Thus, the question surrounding young voters is not so much whom they will support as whether they will officially register that support in the voting booth.

I don't think it's Obama's policies that are turning the young off to politics. They don't pay close enough attention to know what those policies are. Rather, it is the president's personae as a "post-partisan" and "post-racial" leader that they know he has failed to achieve and are both disappointed and cynical about politicians because of it.

Might 2 million or so fewer votes for Obama make a difference? Since the highest turnout states for the youth vote were in blue states anyway, it probably won't matter in the long run.