Here is an interesting article that suggests that the number of self-identified conservatives has increased in recent years:
I believe this number shifts quite a bit over time but this article suggest that exit polls in 2010 reflect a more conservative electorate. (Conservatives - 41%, Moderates 29%, Liberals 20%)
Obviously the weekly standard is a conservative publication and these numbers are based on very high republican turnout in the 2010 elections.
I would suggest that the 2008 numbers (Conservatives - 34%, Moderate 44%, Liberal 22%) should reflect higher democratic turnout and thus be much more liberal.
It is interesting that the percentage of liberals has not changed significantly. This could be a sign that those who identify themselves as moderate probably lean to the conservative side.
Many have suggested the United States is a center-right country. I believe that to be the case.