From First Read:

However, has anyone noticed the biggest upside to both Obama and McCain waiting so long to name their 'mates? It means less time for the media and activists to second-guess their choices. By the time any handwringing over the running mate hit critical mass, the story will be the conventions, the start of the fall campaign, or the debates -- but not the VP's background. Of course, if interest groups cry loud enough, that could be enough to break through the clutter.

Right. If McCain or Obama picks an unconventional candidate, the interest groups will still howl. But it will be a question of resources and timing then: Do they get behind the ticket and rally the troops or do they howl? Probably the former.