As this story points out, the line has been a gag for years. I don't remember where I was, but somebody once said it to me and I wasn't familiar with its origin. Somebody explained it to me. I have since seen the movie many times and would have known the line's meeting had I been on that flight.
My first thought was this is silly. But if somebody doesn't know the origin of the phrase, can you really blame them for being rattled by seeing "prepare to die" on a shirt on a plane? Maybe it's an overreaction but my philosphy on these things is this: people on planes are going to be edgy for a long, long time. Why take the chance?