Parsing McCain's words: It seems as though McCain may have now said that he would oppose or at least vote against) the immigration bill that he had earlier support. See this Hot Air clip (around 30):

HOOK: Senator McCain, let me just take the issue to you, because you obviously have been very involved in it. During this campaign, you, like your rivals, have been putting the first priority, heaviest emphasis on border security. But your original immigration proposal back in 2006 was much broader and included a pathway to citizenship for illegal immigrants who were already here.

What I'm wondering is -- and you seem to be downplaying that part. At this point, if your original proposal came to a vote on the Senate floor, would you vote for it?

MCCAIN: It won't. It won't. That's why we went through the debate...

HOOK: But if it did?

MCCAIN: No, I would not, because we know what the situation is today. The people want the border secured first. And so to say that that would come to the floor of the Senate -- it won't. We went through various amendments which prevented that ever -- that proposal.

However, the CNN transcript offers a different transcription of McCain's response:

MCCAIN: No, it would not, because we know what the situation is today. The people want the border secured first. And so to say that that would come to the floor of the Senate -- it won't. We went through various amendments which prevented that ever -- that proposal.

"I would not" vs. "It would not." It sounds like an "I" to me (see around 00:34 or so)...Also, McCain could be being extra tricky: he "would not" vote in favor of the bill because, he believes, the bill would not come to the floor; you can't vote in favor of a bill that's never on the floor.

Still, McCain seems to have a very hard time saying clearly how he now feels about previous McCain-Kennedy immigration bills, and, just a few days ago, he seemed to say that he would sign McCain-Kennedy if it came across his desk as president. He mainly emphasizes instead that the bill won't come across his desk--without saying what would happen if McCain-Kennedy-like legislation did come across his desk.
Kaus wonders:

P.S.: I don't quite understand why McCain can't just say, "No." (He could then give the same little talk about securing the borders, etc.) Unless, of course, the real answer is "Yes." ...