According to this article:
Southwest fuselage hole prompts Boeing 737 directive
The FAA will issue an AD tomorrow (Tuesday, Sept 14), "proposing that operators of approximately 135 US-registered Boeing 737-300, -400 and -500 aircraft begin performing external non-destructive inspections of a certain area of the fuselage skin for evidence of fatigue cracks every 500 flights".
However, "operators who have installed an external doubler in the area, as specified in a previous Boeing service bulletin, will not have to perform repetitive inspections after an initial check, providing the repair meets certain other criteria spelled out in the proposed AD".