Wow, this is either a statistical fluke or a pretty remarkable turn-around. A week ago, Public Policy Polling, an outfit I hadn't heard of before this year but which has had a respectable run during the primaries, had Obama up only a single point on Clinton in North Carolina. In a poll out today, he's up 21 points. (Thanks to Pollster.com.) Granted, the previous poll was conducted on March 17, the evening before Obama's race speech in Philly, and quite possibly the worst evening of the campaign for him. But still...
For what it's worth, here's the explanation from the PPP write-up:
After a week in which Barack Obama made several appearances in North Carolina and confronted the controversy with his pastor by making a major speech on race, he has expanded his lead in North Carolina to 21 points.
Obama leads Clinton 55-34 in the state. His gains were particularly strong in the Triangle, the media market where his major speech on the war last week in Fayetteville got the greatest amount of attention.
Obama also pulled within a 47-40 margin of Clinton with white voters after trailing Clinton 56-30 last week, an indication that his speech on race in Philadelphia last week may have earned him some points.
Update: For what it's worth, the final PPP polls in Ohio, Texas, Wisconsin, and South Carolina were all pretty close to the mark. And when they've been off--like in Georgia--they've still been closer than most other outlets. (They don't appear to have polled much in Super Tuesday states, or for the Potomac primary. Ditto Iowa and New Hampshire.)