From the Baltimore Sun:

Obama retook the oath before a handful of aides in the White House Map Room—31 hours after he spoke the words before more than a million people arrayed on the Mall. In the first go-around, Chief Justice John Roberts botched the wording, deviating from the language in the Constitution. Following along, Obama repeated the mistake.

"Out of an abundance of caution," White House Counsel Greg Craig said in a statement, Obama decided to retake the oath Wednesday. Roberts again presided.

From UPI.com (United Press International)
Constitutional law experts said the do-over probably wasn't necessary but was done to avoid any gray area.