By JEFF BAILEYPublished: May 6, 2008The big cities that Southwest Airlines has invaded in recent years — San Francisco, Denver, Pittsburgh, Philadelphia and Washington by way of Dulles International Airport — all happen to be air travel markets that were dominated by either United Airlines or US Airways… To read the remainder of this article, visit http://www.nytimes.com/2008/05/06/business/06air.html?partner=rssnyt&emc=rss
