Prof. Ken Katkin writes, though with the preface that this is "just for fun":

(1) The 20th Amendment provides that "[t]he terms of the President and Vice President shall end at noon on the 20th day of January...."

(2) Art II., Sec. 1 Cl. 8 provides that "[b]efore he enter on the Execution of his Office, [The President] shall take the following oath...."

(3) President Obama did not take the Oath of Office until about 12:03 pm today, after Vice President Biden took it at about 12:01 p.m. (Yo Yo Ma and Itzhak Perlman were still fiddling at noon).

(4) Therefore, there was a brief window (just after noon) when George Bush and Dick Cheney were no longer President and Vice President, but Barack Obama and Joe Biden also were not yet qualified to enter on the Execution of their offices.

(5) The Presidential Succession Act, 3 U.S.C. sec. 19(a)(1), provides: "If, by reason of ... failure to qualify, there is neither a President nor Vice President to discharge the powers and duties of the office of President, then the Speaker of the House of Representatives shall, upon his resignation as Speaker and as Representative in Congress, act as President." Section 19(b) states that the President Pro Tempore of the Senate shall act as President (under the same terms and conditions) if the Speaker of the House fails to qualify.

(6) Neither Nancy Pelosi nor Robert Byrd actually resigned their seats in the Congress. Thus, neither of them qualified to become Acting President under the Presidential Succession Act. Plus, interbranch appointments might be unconstitutional anyhow. See Akhil Reed Amar and Vikram David Amar, Is the Presidential Succession Law Constitutional?, 48 Stan. L. Rev. 113 (1995); but see Howard Wasserman, Structural Principles and Presidential Succession, 90 Ky. L.J. 345 (2002).

(7) Section 19(d)(1) of the Presidential Succession Act provides: "If, by reason of ... failure to qualify, there is no President pro tempore to act as President under subsection (b) of this section, then the officer of the United States who is highest on the following list, and who is not under disability to discharge the powers and duties of the office of President shall act as President: Secretary of State ...."

(8) Notably, Section 19(d)(1) does not condition the Secretary of State's assumption of the powers and duties of the office of President on resignation of her current office, nor does elevation of the Secretary of State raise any constitutional issue of interbranch appointment.

(9) The term of office of the Secretary of State does not automatically terminate at noon on the 20th day of January.

(10) On January 20, 2009, Condoleeza Rice was (and is) still the Secretary of State.

(11) Accordingly, from 12:00 noon until 12:01 p.m. (when Vice President Biden took the oath of office and became Vice President), Condoleeza Rice was momentarily the Acting President of the United States, our first African-American President.

I suppose the obvious counterargument is that Secretary Rice also never took the Oath prescribed in Art. II, Sec. 1, cl. 8, and thus was no more qualified than Barack Obama or Joe Biden to act as President at 12:00 noon. But if Secretary Rice was not President from noon to 12:01, then who was?

See also the Washington City Paper blog, which first blogged Prof. Katkin's analysis (though please note that I have Prof. Katkin's permission to reprint his thoughts). To be sure, I think Orin Kerr is right that under the Twentieth Amendment that "the terms of [the incoming President and Vice President] shall then begin" when the outgoing pair's "end at noon on the 20th day of January," so Pres. Obama was probably President starting at noon, even before the oath was taken. But Prof. Katkin's analysis is more fun.

UPDATE: Prof. Katkin now reports that Biden took his oath of office before 12 noon, despite earlier press accounts, so all the above is moot. Another beautiful theory destroyed by ugly fact.