What's your read on it? As I see it the electors can vote for anyone they want, if BHO is discovered to be ineligible they would probably rally around Biden or HRC. If nobody gets 270 votes the decision falls to the House, which again would result in either President Biden or Clinton being sworn in.
If Obama is sworn in and later found to be ineligible, if he doesn't resign, he would have to be impeached by the House and removed from office by the Senate. (Not holding my breath. I suspect I know how senators McCain, Graham & Franken would vote.) If they did remove him from office, Biden becomes President.
The rules are different from state to state as to whether an elector can, or cannot choose to vote as they wish. But, here's a start to an answer, assuming that Obama is found to be ineligible after the electors vote:
From the US Constitution ~
Article II Section 1. ..."The President of the Senate shall, in the presence of the Senate and the House of Representatives, open all the certificates, and the votes shall then be counted. The person having the greatest number of votes shall be the President;..."
McCain and Obama were not the only candidates in the race. The electors haven't yet voted, so we don't know who they will actually vote for, but there is a possibility that one of the lesser candidates, such as Bob Barr, will receive some electoral votes.
If both McCain and Obama are ruled to be ineligible under Article II of the Constitution, their electoral votes would have to be set aside as disqualified by the House and the Senate. This leaves Barr with the probable highest number of qualified electoral votes.
As I said, I'm just taking a stab at this. I know that there is other language in the Constitution which relates to this issue, but this is one take on it.