I wonder what effect the merging will have on expiration. I have a small amount of miles at Frontier, and they sent me an email that they would expire soon a couple months ago at least, and my last activity was in Sep 2008, yet they're still there as of yesterday. Meanwhile, I have a single-digit amount of Midwest miles, from about the same period I think but maybe a few months later (I got the Midwest credit card at one point, transferred all the sign-up bonus miles to Amtrak, and so only the original qualifying first purchase miles were still there after that!).

I wonder whether they'll try to keep the original last activity dates from whichever one is newer, or whether they'll use the merger of miles from two accounts itself as "activity"? (I can't find anything in the T&Cs or merger FAQ to explain how the merger will affect expiration of miles, if you had miles at both Frontier and Midwest.)