gydell,
Not sure how we got it. I know when we had the first major change (on the outbound) I told them for the inconvenience that they should compensate us by giving us assigned seats because they never notified us about the change. The agent said I would have to talk to customer relations. I never did.
In the interim, our flights were changed numerous times.
Then, a couple of weeks ago, we got a call from AirTran telling us our flight had changed and that we should call them. They also said we'd get an email from them. I checked online and the flight times didn't change from the last time I checked, so I thought we totally got moved around. Then I went into our reservation and it was still the same. However, this time I could see the seats were assigned going down (I would have rather had the return flights assigned since we'd be on the boat the day before). So I called them, since they told me to, and asked the agent if anything had changed. She said no. Then I asked her about the seats, and she said they were all assigned already.

So I don't know if anything ever got noted on our record that we requested assigned seats due to the inconvenience, or they just did it because of the change. The only other thing I can think of is that we booked so early, and now that the schedule was set, they decided to assign seats. Either way, we're happy.
At least with AirTran you can get an assigned seat if you check-in online 24 hours prior. On Southwest, you have to show up at the airport, get a boarding group (I believe they have five) and then it's take what you want when you get on board. If you board in the last group, you get what is left. Furthermore, Southwest always overbooks their flights. If you watch those airline shows on cable, not sure of the name of it, one of the problems is always dealing with overbooked passengers. We will NEVER fly Southwest no matter how cheap they are. At least not with a child.
All I can say is check your reservation about a month or two out and see if they assigned you a seat. Ours were assigned about 8 weeks out.
The other thing is, go online the day before the flight and check-in. If you can, try to check-in prior to 24 hours. I'm not sure if they are on a hard 24 hour or just the day before. Unfortunately, we'll all be enjoying
Castaway Cay and won't be able to check-in exactly at the 24 hour mark, but I think I'd rather enjoy the sun

and sand than planning my trip back to the cold.
Jeff