Thanks for the info everyone --- I knew about the pet policy and fees --- it was the underseat height I was after. Sent an email to Matt at Seat Guru -- he had never had that question before and thought that the heights might be standard on the larger jets....

Well as luck would have it, I had 2 flts. today (conex thru PIT) - one an A321 and the second a 737-400. I brought a small tape measure and can report that the heights are not the same -- 13" on the 321 in first directly in front of the seat - 14" in the middle - and 13" again in front of the window seat. On the 737-400, the height is 12" all the way across. Of course I told the FA working A what the heck I was doing (am sure she was thinking it was just another crazy CP) -- she thought that for soft pet carriers, the best seats with greatest height would be in coach on 319...

Bottom line -- think the lil guy just might fit!