Section 3. ..... If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President elect shall have failed to qualify, then the Vice President elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President elect nor a Vice President elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.
So, if Obama was deemed as 'not qualified' then Biden would "act as until a President shall have qualified ..."
Very relevant ... I am glad that it does not say Biden 'shall become' !!!!
That will only have bearing if Obama is not thrown out as the “President-Designate” prior to the EC meeting to decide the “President Elect.”
If Obama is thrown out before then, all bets are off. States could meet to appoint EC representatives as they saw fit to go to the EC meeting. My bet is that McCain would get his minimum 270 Electoral votes to be President, but then his own “natural born” issue would be challenged.