Thanks to the wiki page on hamburgers, I've learned a lot about the zillion regional variations, but it seems factual evidence eludes me on this one, so I'm looking for anecdotal evidence.
Rumour has it that American visitors to Canada are weirded out by the fact that we put mayonaise on our burgers (something which I was not aware of until I actually looked in one the other day, actually.) I think that's a pretty good base for a scene where a bunch of Americans (and one lonely Canadian,) stop at a small local burger joint for lunch and someone happens to notice that there's a little something extra on his burger.
But, is it realistic? In your local-burger-joint experience (I'm ommiting McDonald's and Burger King on this one,) do your run-of-the-mill American hamburgers never have mayo on them? Do Canadian burgers always have mayo?
ETA: Holy guacamole everyone! I left for work for only a few hours and wasn't expecting to come back to a gold mine! Thanks so much for your answers!