We love SWA. The staff is very friendly and efficient. They do allow preboarding with an infant. Make sure you ask at the gate where to stand for preboard because when I flew with my 6 month old, we had to wait in a different area (this could have been because of the way the airport was set up).
One other thing that I thought was strange. They asked me for my son's birth certificate when I checked in. Of course I did not have it and she said, "she would let it go this time." I have never been asked for any of my childrens' birth certificates before or after this one incidence, so I'm not sure about this policy. I would call and ask SWA about this. This happened before 9/11.
Also, SWA seems to have a lot of single business travelers (who are willing to trade seats so a family can sit together).