I don't expect SW to follow the legacy airlines by charging for the second bag any time soon. Until mid 2007 - you could actually check THREE bags on Southwest......besides, I don't think they need ANY more publicity....especially after last week on Capital Hill.
I fly SW all the time -- my preferred airline. They have far fewer $$$ games (fees) than the legacy airlines. Delta seems to be the current leader in fees to...second bag, overweight bag, checking bag at the curb. Southwest doesn't play these games.