Near as I can figure, the NY Times popped the question first:
McCain’s Canal Zone Birth Prompts Queries About Whether That Rules Him Out
The question has nagged at the parents of Americans born outside the continental United States for generations: Dare their children aspire to grow up and become president? In the case of Senator John McCain of Arizona, the issue is becoming more than a matter of parental daydreaming.
Mr. McCain’s likely nomination as the Republican candidate for president and the happenstance of his birth in the Panama Canal Zone in 1936 are reviving a musty debate that has surfaced periodically since the founders first set quill to parchment and declared that only a "natural-born citizen" can hold the nation’s highest office.
I first heard about it on a local Fox station's morning program.
The way it was presented there made it sound as if the DNC raised the issue. The "morning anchors" were talking about the issue as if either the Obama or Clinton campaigns were making noise about filing some sort of legal challenge to McCain's candidacy. They further opined that it would be stupid for the "Democrats" to do so because they would be going after a "war hero" and that just wouldn't sit well with anyone.
Is this a new talking point? I never even heard about McCain's possible problem from any trusted news source until I went searching for it this morning. I would assume that if the Democratic party were looking to challenge McCain's eligibility that would be major news. Seems like not so much.
The NY Times gives a good summary of the issues, and most other publications I've seen are just running the same story. There's some analysis out there, including a reference to an "interesting political analysis" on Redstate. I just can't bring myself to go look at that one.
My conclusion is that he's eligible and nobody with any standing would bother challenging his eligibility. There's no question that the Panama Canal Zone was under U.S. sovereignty when McCain was born there. I would say that's analogous to being born on a U.S. military base or embassy outside the country - it's still U.S. soil and that should make you a "natural-born citizen".
That said, the issue has apparently never been litigated before so there's no real precedent.