From a press release from the White House, a statement from White House counsel Greg Craig:
We believe that the oath of office was administered effectively and that the President was sworn in appropriately yesterday. But the oath appears in the Constitution itself. And out of an abundance of caution, because there was one word out of sequence, Chief Justice Roberts administered the oath a second time.
Well, damn. That puts one wingnut meme to rest pretty quickly.
Update: And from the pool report, a first-hand account of the riveting event (typos and all):
At 735 pm, Roberts administred the oath of office again to obama in the map room. Robert gibbs said the wh counsel, greg craig, believes the oath was fine Tuesday, but one word was out of sequence so they did this out of a "an abundance of caution."
"We decided it was so much fun..." Obama joked while sitting on a couch.
Obama stood and walked over to make small talk with pool as roberts donned his black robe.
"Are you ready to take the oath?" Roberts asked.
"I am, and we're going to do it very slowly," obama replied. Oath took 25 seconds.
After a flawless recitation, roberts smiled and said, "congratulations, again."
Obama said, "thank you, sir."
Smattering of applause.
"All right." Obama said. "The bad news for the pool is there's 12 more balls."