If it happened before the Electoral College voted, they could vote for someone else entirely. With how it's done now I believe, but am not certain, they could not vote for Biden as POTUS. Another candidate would have to be chosen much like happened when Horace Greeley died and his electors split votes among different candidates. It didn't matter since Greeley was the loser. This year, however, it could matter. We could end up with Hillary installed as President.
If it happened after they voted and before the inauguration, the House could decide who should be president but they are limited to only the top 3 electoral vote getters. With Obama disqualified, McCain would surely be president and likely stuck with Biden as his VP.
Biden would be Acting President, with Obama being essentially a figurehead. Section 3 of the 20th Amendment makes this clear.
O-Biden McCain's VP, makes no sense to me???