Ed Morrissey may have something here about Barack Obama's decision to go after Bill Clinton's largely misleading statements about Obama's record:
Obama made a big step in making this an issue. He has explicitly tied Bill to Hillary at a time when other Democrats have begun to beg Bill to get more in the background. Newsweek has an article in this week's edition that name Ted Kennedy and Rahm Emanuel as party leaders who have told Bill directly to pipe down. Obama has now made it harder for Bill to withdraw from the fight, and Obama clearly now believes that Bill could be an effective way to attack Hillary.
Clinton may face disadvantages as a woman running for high office, but she garners advantages for the same reason. One is that when competitors attack her, she looks more like a victim that a male candidate would. Contrast how much Mitt Romney benefited (not much) from the gang beating he took at the first New Hampshire debate with how much mileage Clinton gained from the relatively light one she received up there for at least one data point.
That said, the more Bill Clinton speaks, the more he makes this nomination fight a referendum on him instead of his wife. Perhaps more important, Obama can go after him with less risk, as he won't be attacking Hillary directly.
It will also be very hard for Bill to not respond. It's not in his makeup, as the vast weight of evidence of his public life would indicate.
This strategy carries significant risk, but it's hard to see where Obama has another choice if he wants to win. His omnipresent challenge is to do it without appearing negative, something he's mostly finessed beautifully so far in this race.