I also asked about this a while back. The reason that SWA will not have aircraft bigger than a 737-700 is that with the higher capacity of the -800, an additional F/A would have to be aboard. That would conflict with thier current payroll philosophy.
So for the near term anyway, don't expect to see any SWA -800/-900 series 737's.
I'm not a real aeronautical engineer, I just play one on Airliners.net.