Well, looks like there's a little more to it than a couple of whistleblowers.
This is from the Wall Street Journal, so I'm going to post it since you have to register and pay to get it.
The Federal Aviation Administration, imposing the largest financial punishment against an airline in about two decades, proposed a $10.2 million civil penalty against Southwest Airlines Co. for flying passengers in 46 of its planes without complying with mandatory inspections to check for possible structural cracks.
"The FAA is taking action against Southwest Airlines for a failing to follow rules that are designed to protect passengers and crew," said FAA Associate Administrator for Aviation Safety Nicholas A. Sabatini. "We expect the airline industry to fully comply with all FAA directives and take corrective action." According to the agency, the airline flew hundreds of thousands of passengers in the planes from June 2006 to March 2007 without complying with a September 2004 safety directive to inspect for fuselage cracks. After advising the FAA of its mistake, the airline received verbal approval from the local FAA Dallas office to keep operating the aircraft, and the FAA said in a press release that the carrier flew an additional 1,451 flights before completing the inspections. The FAA said the size of the penalty "reflects the serious nature of those deliberate violations." Southwest has 30 days to appeal.