Southwest Airlines flight WN2592, a Boeing 737-700 (N913WN) and Southwest Airlines flight WN3944, a Boeing 737-300 (N650SW) collided during pushback from the gate at Philadelphia International Airport, PA. The left wing and horizontal stabilizer of N913WN contacted the right winglet and horizontal stabilizer of N650SW. chad been previously pushed back and awaiting taxi clearance at the time of the incident. The tug operator reported he looked away at an uninvolved airplane and when he looked back, the wing walker was signalling him to stop, but he was unable to stop prior to impact. N913WN was substantially damaged due to the impact to the wing spar. N650SW had minor damage. There were no injuries to the 10 total crew members, 215 total passengers, and 2 ground operations personnel Probable Cause: the failure of the ground operations personnel to maintain awareness of the relative position of the two airplanes.