Take a look at this page from AirTran's site:

http://www.airtran.com/f9program.aspx

It appears that, if you're on a Frontier codeshare flight that connects in Milwaukee, you can earn AirTran flight credits.

I haven't tried to do it, but I find the whole notion of earning A+ rewards on flights potentially operated by Republic and/or Midwest pretty hilarious. Maybe there's something in their system that prevents folks from doing this.

I've got to think that this arrangement will be ending soon, but so far there's no evidence of that.