The aircraft involved is Boeing 737-3H4 N632SW (cn 27707/2799), delivered 06/13/96. Not a very old aircraft, but a well used aircraft, with lots of short cycles on her. I think it is possible that this is more a case of metal fatigue then anything else, given statements like in avalons post:
"Clear evidence that the skin separated at the lower rivet line".
Read the opening paragraph of the article avalon linked to:
"Investigators found "widespread cracking" on the skin of a Southwest Airlines Boeing 737 that made an emergency landing in Arizona after a hole opened on top of the aircraft during flight Friday, a National Transportation Safety Board member said Sunday."