Because he was born in the Panama Canal Zone when his father was posted there for the Navy, McCain has a little more legal work to do than most other presidential candidates to make sure he's actually eligible to serve if he wins the election. "It is not a slam-dunk situation," Catholic University law professor Sarah H. Duggin told the Times. That's because the Constitution says only "natural-born" citizens can make it all the way to the White House. (Which may be the only thing standing in the way of Arnold Schwarzenegger's triumphant cross-country march from Sacramento to 1600 Pennsylvania Ave.) Legal scholars agree that means naturalized citizens aren't eligible, but no one has ever litigated the question of what the provision means for someone who wasn't born in the continental United States, but who had citizenship at birth because of his or her parents, or because of laws giving citizenship to children of U.S. military personnel posted to the Canal Zone.
The issue has actually come up a few times before; Barry Goldwater (the last Arizonan to win the GOP's nomination for president) faced some questions because when he was born, Arizona wasn't a state yet. Mitt Romey's father, George Romney, was born in Mexico, but his 1968 presidential campaign didn't last long enough for anyone to worry about whether he could serve.
McCain's campaign has apparently gotten concerned enough to have heavyweight Washington lawyer Ted Olson look into the matter. (It inherited his support when Rudy Giuliani dropped out and endorsed McCain.) "I don't have much doubt about [McCain's eligibility],” Olson told the Times, though he is apparently still researching the issue. While it's unlikely the Democratic National Committee would pursue the matter, don't be surprised if some lawsuit gets filed by someone that McCain's lawyers will have to fend off using Olson's brief. After all, it's a long way until November.