According to the NTSB (National Transportation Safety Board) and the referenced Marketwatch article below, Boeing and not Southwest Airlines is responsible for the cracks and holes in the fuselage of the Boeing 737 airplanes. At issue was misaligned rivets and loose bolts. Boeing will need to address quality control as it ramps up production to meet ever-increasing demand for its airplanes.
Boeing 737 airplanes are flown by the following airlines: AirTran, Alaska, American, Continental, Delta, Miami Air, Southwest, Sun Country, United Airlines, and US Airways in the United States, and hundreds of international airlines according to Wikipedia.
In a separate development, a Boeing 737 airplane flown by Southwest ran off the runway at Midway Airport. The likely cause was water on the runway, as the airport had been experiencing heavy rain yesterday morning. The pilot may have saved the airplane from crashing through the airport walls by veering into mud on the side of the runway.