If you're following this story on the internet, it's pretty crazy. There's constant speculation, definitive-sounding leads, contenders ruled out, ruled back in, ruled out again, with darkhouse names coming in and out. The assumption is that the McCain camp is doing a great job of hiding a secret (and stepping on the Obama convention story.)
But maybe they really don't know who it is yet? (As of, say, 8:00 AM.) It sounds crazy, but hear me out. They had a definitive date for the announcement set out well in advance. But McCain's notoriously faction-laden team has been reportedly at odds over the veep pick. (Outside advisor Karl Rove even tried to go around McCain to subvert Lieberman, who is known to be McCain own preference.) We also know that McCain's decision making has a seat-of-the-pants style. And we know (per this Politico report) that the Biden choice, which was less than a week ago, scrambled the calculations for McCain.
So, maybe they really can't agree. Maybe there's still a last-minute back-and-forth. That would explain why more than one contender may (or, depending on what report you hear, may not) be in or on their way to Dayton now.
Crazy, I realize. But crazier things have happened.