LAT/Bloomberg national poll:
http://www.latimes.com/news/printedition/front/la-na-poll27feb27,1,7063277.storyObmama 48%
Clinton 42%
Interestingly, they also give the results for the subsample of people living in states that haven't voted yet:
Also, the wording of the question is basically "Who will you / who did you vote for?", so if the person has already voted, the question is asking who they voted for. But when the question is asked as "Regardless of who you already voted for, who do you now support?", it's:
Obama 55%
Clinton 37%
So it looks like many of the people who voted for Clinton are now backing Obama.