On FindLaw.com today, law professor Michael Dorf weighs in on the oath-of-office controversy. I was pleased to see that he reached pretty much the same conclusions as I did: that because of the Twentieth Amendment, the oath wasn't necessary to make Obama the president; that although it might call into question any acts he took between noon on January 20th and the re-oathing, that is pretty unlikely; and that he didn't do much during that time anyway (two executive orders, which he could just sign again).
Dorf also notes that while there might be some alterations to the oath that would render it ineffective, assuming that mattered, the minor errors on the 20th didn't change its substance. Or, as he puts it, "a President who stated 'Yippee, I'm President' . . . could not reasonably be deemed to [have satisfied] the constitutional requirement."
Since even George W. Bush did not say anything like that, I think we can assume it is very, very unlikely.
Link: Michael C. Dorf, When Did Barack Obama Officially Become Eligible to Act as President?, FindLaw.com (Jan. 26, 2009).