I'm going to go out on a limb and say that Romney is going to pick Rob Portman for his VP pick. Why? Because without Ohio Romney can't win.
Mitt desperately needs to win Ohio and it's 18 electors. Ohio means he can loose NV or CO. Iowa tends to lean democratic as well. Even if Mitt does pick up Ohio he MUST get either one of the aforementioned three (IA, CO, NV) or he STILL looses by 1-2 votes.
So, unless the Romney camp has some internal polling that says they can win Ohio on their own, Portman is the guy. If the Romney camp think that they have Ohio sewn up then look for Brian Sandoval (Gov. New Mexico).