Aviation Week article mentiones that Southwest would be a launch customer for a new Boeing narrow body aircraft.
http://www.aviationweek.com/aw/generic/story_generic.jsp?channel=aviationdaily&id=news/avd/2010/05/20/02.xml&headline=Analyst: Boeing Will Opt For New Narrowbody
http://www.aviationweek.com/aw/generic/story_generic.jsp?channel=aviationdaily&id=news/avd/2010/05/20/02.xml&headline=Analyst: Boeing Will Opt For New Narrowbody