Passing 18;000 FT during climb; a B757 First Officer is informed by a Flight Attendant that a passenger has seen part of the left wing leading edge come off the aircraft. After consulting with maintenance; it is determined that a previous repair may have failed and flight diverts to nearest suitable airport.

Narrative

On climbout passing 18;000 FT; the Flight Attendants called the flight deck to advise us that a passenger had observed a part of the left wing leading edge had come off the plane. We did not have any control issues at this time; and we asked if the passenger had any flight experience; or any knowledge that would help detail the problem. He identified himself as an FAA General Aviation Inspector. Upon level off; I (the First Officer) went back to talk to the witness; and inspect the wing. There was a hole in the top of the leading edge of the left wing. We called Dispatch and Maintenance; and discussed the problem. Maintenance advised us that a previous repair to that leading edge must have worked itself loose. ZZZ was chosen as the best divert choice from our current location. We declared an emergency with ATC; and they gave us direct with some deviation for weather along the way. Dispatch sent us all the appropriate paperwork for the diversion; and we set up for the arrival and landing. The landing was uneventful; the fire trucks inspected us upon arrival; and maintenance met us at the gate. We submitted the report for the overweight landing; and gave maintenance a verbal briefing in addition to the required paperwork.