Not specifically covered by the Constitution. The most logical and constitutional approach would be to have Biden confirmed as VP by the Senate, and if the president-elect is unable to serve, for whatever reason, the VP succeeds to the presidency.
True. I was thinking if Obama was found not eligible before he was sworn in that he could not take office and his election would be held null and void since he was not eligible. Rhomney would be the next in line by vote count.