Well, since the country hasn't gone through this before, it's a legitimate question, but I think Joe will be out of luck. The Obama-Biden ticket was not a valid presidential ticket, so votes for the Democrat nominees ought not to count. Votes for McCain count just fine, so the GOP got virtually 100% of the legal votes in the 2008 presidential election.
But that's not how it works. You didn't vote for the President and Vice-President, you voted for members of the Electoral College. In turn, they voted for the positions of President and Vice-President, each on a separate ballot. Biden's election to the Vice-Presidency is completely separate from Obama's election to the Presidency. The fact that politics dictate that the two are selected as a pair does not change the underlying Constitutional process.
So where does the Constitution say you have to vote for a valid presidential ticket? The electoral college could cast 535 votes for Bullwinkle the Moose and there is nothing in the Constitution preventing it, nor would those votes be invalid. Bullwinkle couldn't serve as president, of course, because even though he is a natural born citizen from Frostbite Falls, Minnesota he is also totally imaginary. And unless his running mate was an actual person qualified for the presidency then the Speaker of the House would become president and appoint her own vice-president with the consent of Congress.