I totally disagree. Just because a movie came out in 1980 and I haven't seen it in 2011 does NOT mean that I'll either never see the movie or have already heard/seen spoilers at some point already.
I took half a dozen film classes in college, and many of the movies I watched there I had never heard about until I took that class. One of them was Metropolis - it came out in 1927, but how many people in the world have actually seen it? There are additional movies that professors recommended we watch if we had the time or inclination that are on my To Watch list but I haven't been able to hunt down or make time for in the past few years. That doesn't mean I never want to watch them and that I won't watch them. It means I have a Very Long List, and Netflix only has so many of them available on Instant Watch.
Now, having said that, I'm not going to insist that there be spoiler markup on every question that talks about the ending of a movie. As Alonzo mentioned, there are (at least should be) other clues that allow people to avoid questions they don't want to be - tags are useful for that. As has been stated in other discussions elsewhere on the network, spoiler markup is a courtesy thing. I do think if someone uses spoiler markup, no one should go back to edit it out, even if you personally think that there's no reason for it. If the OP felt like being polite, let them be polite.
A separate issue is the one of the entire body of a post being spoiler markup. But that's a different conversation, taking place on a different meta thread.