I have heard someone tell a story about that happening once. They said that the cabin crew announced that they would be playing a game; they would call out an item, and give a free drink to the first person who could find that in their carry on. The first couple of items were fairly standard things; "a fountain pen", "a camera", etc. Then they got to "a commercial pilots license". Someone actually did raise their hand, and the cabin crew escorted them up to the cockpit.

There was never any problem with the flight, and he never found out what happened. Probably something like the co-pilot had a problem that prevented them from continuing to act as the co-pilot, and they wanted to maintain sufficient redundancy just in case.

That said, pilots -- or even non-pilots -- can actually do fairly well landing a large aircraft as long as they're being talked through the process by someone experienced.

See http://mythbustersresults.com/episode94

Unassisted, perhaps, but what about with radio instructions? Modern aircraft with autoland systems mean that if you can punch buttons you'd probably be okay, I'd think. It might not be 'by-the-book', but looking at a video of the process it doesn't seem too complex: http://www.youtube.com/watch?v=LIaMALJjOEc