I think once something airs on TV, it’s no longer a spoiler. It’s just what happened.
It’s courtesy to wait a bit for the West Coast, but if they’re bright enough, they’ll know to tread cautiously online until they’ve watched something big. I’m thinking Lost, House, ER finale, etc.
I think it’s fair to wait until a movie is available on DVD for a bit before assuming everyone who wants to watch it has. I think it’s proper to say “Spoiler Alert” when talking about a movie twist.
I think twist endings in books should always be properly notated. Books don’t generally have must-see-it-now appeal, so it’s always a potential spoiler.
It’s unfair to force people to not discuss something fun and current, like “last night’s Lost,” simply because you were too tired to watch it. Yes, it will ruin the surprise. If it’s important to you, either watch it or remove yourself from the conversation.
It’s not a spoiler once it aired. It’s just what happened.