I strongly suspect you're right, Kevin.
It's a massive, hammer blow to Cameron who must be in tortures over this.
Boris just doesn't seem to generate the "loathe" feeling in people.
He's far from being the "bumbling oaf" of which he is often accused - and despite his privileged background, he's still viewed as a "man of the people" (which, of course, he isn't).
It'll be interesting to see just how vocal he will be over this. He said it was only after a great deal of soul - searching that he reached this decision and how much he hurts over this seeming act of disloyalty to the leader of his party.
Et tu Boris.