One bit of context: Fox News is pretty obsessed with Ayers. The last few times I've checked in on Sean Hannity, he's been blathering away about him, and a Fox crew even ambushed him recently for an impromptu interview. Obama was going to field that question sooner or later, and in that light I don't see why it should be off-limits until the general.
P.S. Hillary did take the low road in her answer in at least one way: She noted that Ayers had said unrepentant things about his bombings in a newspaper article published on September 11. Needless to say, though, that means the interview was given prior to 9/11. So the actual date of publication is totally a totally irrelevant point and she's just demagoging 9/11 here in a way that would make Rudy Giuliani blush.
P.P.S. A 2001 NYT article on one of those Weathermen pardons by Bill Clinton, complete with--harmonic convergence!--a "sickened" Bernie Kerick.
Update: My own mental commenter is asking, 'So now everything Sean Hannity's going on about is fair game for a debate question?' Well, no. But I still think this one was coming eventually, and wasn't totally out of bounds, even if it's highly tangential to what sort of president Obama would be.