Obama really hasn't taken a 'fundamental' position on foreign policy. He made a few comments in the debates I disagree with, but the foreign policy speech he gave in Chicago I more or less agree with - the U.S. must stay engaged in the world, etc..
But presidents do change their minds once in office. GW Bush was more a realist, anti-nation building guy in '99 and look where we are today - building nations in Afghanistan and Iraq.
If Obama gets elected he'll realize he can't just quickly withdraw from the Middle East and wish the Iraq war never happened. He'll have to deal with the situation as it is, and I think he' sharp enough to do that.