According to the Consitution, Obama became the 44th President of the United States at noon on 20 January regardless of whether the oath was taken or not.
However, to dispel any confusion or future court cases, and questions that Obama might not be a President legally, Barrack Obama retook the Oath of Office in the white house yesterday.
Just to recap the oath that took place on Tuesday:
Justice Roberts: .........that I will execute the office of President to the United States faithfully....
Obama: .......that I will execute......(pause, realizing the mistake and probably deciding whether to follow)
Justice Roberts (simulatenously with Obama): ....the off....faithfully the pres......the office of the President of the United States....
Obama (simultaneously with Justice Roberts): ....the office of President of the United States faithfully....
This must be the blooper of the year. :)