While I believe it'd be a smart move economically to merge the 13in MBA with the 13in MBP, there's probably still a certain market out there that'd like a reconfigurable, upgradeable pro-type 13in Macbook.
Thusly, I see the Pros getting updated first, then the Airs getting updated later. Most likely because I believe the LV CPUs would come out before the ULV CPUs. Also, they need to space out launches to keep everyone psyched and begging for more (i.e. iMacs/Mac Pro get updated first, then MBP and Mac Mini a month later, then the Airs a month after that, then the iPhone a month or two after that, and then some new iPods, and then before you know it, the 4th gen iPad)