GordonFreeman said: Citizen Kane came out way before that and there are plenty of people who haven't seen it, but may in the future. Spoilers are always spoilers.
There's a certain nebulous cutoff point to when a spoiler becomes common knowledge though.
Vader is Luke's dad, Leia is his sister. The boat sinks. It was his sled (Which happens to be the name of the TVTropes article dealing with exactly this topic)...The cutoff date varies depending on certain things.
I can assume that within short order, *everybody* knew what happened to Dumbledore, for instance...That quickly stopped being a spoiler and entered the public subconscious.
Spoilers are not always Spoilers. Spoilers have a lifespan. It's an indefinite one, but a lifespan nonetheless.
