[seton89]

Article 2, Section 1: that the Vice President is the direct successor of the President.

You are assuming that 0bama is the President. If he is ineligible for election, he never was president. The eligible candidate with a majority of valid votes becomes president. McCain.

Biden would still be VP.

[okie01]

You can contend that Obama was never president. But he was -- even if he was subsequently proven ineligible. And Biden was always vice-president. In the event of a vacancy, he becomes president. The Constitution is quite clear on the matter.

Betcha. McCain would not be a consideration, as he was never elected. Note that the entire line of succession gives preference to those who were elected.