Is it too early to start the veep speculation on the Republican side? Probably, but what the hell. A McCain-Huckabee ticket gets bandied about often, and I've always assumed it made sense. But Karl Rove was on Fox earlier tonight and shot the idea down pretty decisively, calling it "double trouble"—i.e., many of the same conservatives who dislike McCain aren't thrilled with Huckabee, either (I assume he's referring to both the talk-radio crowd and the Club for Growth-types who are only barely persuaded that McCain's recent return to the supply-side fold is sincere).
Whatever other names one might call Karl Rove, he presumably has a sense for where the base's erogenous zones are, so I guess I'd put a fair bit of weight on his judgment.