I might be over-analyzing. I think more than 270 Obama electors are required by State law in each State to vote for Obama. IOW, they have to vote for him even though he would be ineligible. A conundrum.
On 3rd thought, in that case, we’d probably have Acting President Biden.
Many if not all of those state laws have "if unable to serve" type of exceptions. Some of those, and others only require voting for the candidate of the party they were elected to vote for, which makes it highly likely that another Dem would be selected by the DNC should The One be DQ'd before the electors vote. If it's after they vote, it seems to me highly likely that one, from the states where it is allowed, will have voted for someone other than McCain or Obama, "just in case", and then that person would selected by the overwhelmingly Democrat