Fred Barnes makes a good point about the advantages Hillary has of using Bill as her attack dog:

When every candidate except Hillary wants to put out unfavorable information about an opponent and be sure to draw heavy press coverage, the candidate himself must handle the task.  And there's a downside: the candidate is deplored for "going negative."  But if an aide or supporter is assigned the task, the media is likely to yawn and the information the candidate wants to trumpet gets far less coverage.

But not in Bill Clinton's case.  He's the one supporter of a candidate whose words are reported to the world under blazing headlines.  Thus when he criticizes Obama on Iraq and other issues, as he did in New Hampshire, we hear about it.  And when he scolds the press for giving Obama a free ride, we not only hear about it but the press takes the criticism seriously.

Without a surrogate like Bill, Obama is at a disadvantage.  He's been wary of responding to charges and criticism by Bill and other Hillary backers because it would detract from the positive tone of his candidacy.  For the same reason, he's been reluctant to go after Hillary himself.  Obama's strategy has been to stay above the fray as much as possible.

Now he doesn't have that luxury.  To counter attacks by the Clinton camp effectively, Obama will have to step forward himself and respond.  And if he wants to be sure voters hear about Hillary's shortcomings, he's the one who will have to point to them.  What Bill does for Hillary, Obama must do for himself.

I think what we are starting to see come into greater focus is the fact that it is not Hillary Clinton running for president—it is The Clintons.  They have gone from distancing themselves from this rumor, to slowly embracing it, to now practically campaigning on it.  If you doubt, just watch this video of Bill Clinton's Nevada victory speech—er, Bill Clinton's "introduction" of his wife after her winning the caucuses there.

With this dynamic in play, it is not only valid, but the responsibility, of their opponents to start asking exactly what role is Bill Clinton going to have in the White House.  Would he have a formal policy role?  How could Hillary's Vice-President, for example, vie for position as #2 with Bill standing by at every turn?  A better question might be, who on earth would want to be her Vice-President in this situation?

If we're really going to be in the business of coronating a wife-husband team as a co-presidency—an unprecedented situation in our nation's history—can we at least start calling it that, and debate it for what it is?