I guess Rick
figured that now that Newt is dead, giving the corpse some oxygen won't reanimate him, and he has more to fear from Mittens, so he is willing to go back into the debate ring, after previously joining Mittens in stiffing Newt and just saying no. Good call Rick.
Mittens no doubt will await developments before deciding. I doubt he will want to listen to Rick talking about him being Obama lite, and Romneycare = Obamacare, etc., unless he figures that the alternative of sending his regrets portends something even less pleasant, be it the resentment of Texas voters or whatever. Maybe he hopes that if he stalls, the debate without his presence will be cancelled. We shall see.