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."
According to the pool report, Roberts re-administered the oath to Obama in the map room at 7:35 p.m.
Just for the record, Obama jumped in too soon yesterday--after Roberts said "I, Barack Hussein Obama" instead of waiting until the chief justice completed the
full line: "I, Barack Hussein Obama, do solemnly swear". Then Roberts misplaces the word "faithfully" in the next line and there's some more confusion. It really isn't that big of a deal, but I imagine if George W. Bush had done exactly what Obama did the news coverage of this gaffe would have been relentless. Thankfully, the media have been able to rise above such pettiness on this occasion, at least for a Democratic president.