Well, it's always a spoiler, but after a certain amount of time, I don't think you need to be careful about not spoiling a movie. For me, once the movie's been out on DVD/Blu-Ray for a while, it's fair game.
Then again, I usually prefer to not get into specific plot points in a film unless it directly relates to a point I'm making. You never know if someone's listening who's never heard of the movie before - I'd hate to ruin it for them before they even get a chance to watch it. I guess it depends on whether I'm talking about a movie in a conversation or on a podcast. On a podcast, I try to tread lightly no matter how long the movie's been out.