I would say that, for theatrical releases, once the film has been released, a simple "Spoilers" alert in the title should be enough to keep the people away that haven't seen it yet. If you still go in and read something you shouldn't have, it's your own damned fault.
Pre-release is where it gets a little fuzzy. Speculation, especially if correct, can be spoiler material, or if you got a "leaked" script and posted about it six months before release.. a simple "spoiler" tag in the title isn't really that useful if no one can reasonably be expected to have seen the film yet.
The discussion came up in the Prometheus thread recently, and someone said that the word "details" in the title meant there was potential spoiler material, but that title was made before they even started filming, it was meant to be
production details, not story details. Actors, locations, things like that, not "Snape kills Dumbledore" material.
I would say the best thing to do would be to leave spoiler material out of thread titles and unmarked threads for at least a year after release, or at least have the decency to use the tags.
That said, once a film hits theaters, there should absolutely be a big honking "(Movie Title) Discussion - SPOILERS!!!" thread opened, and it should not be the same thread that existed prior to release. And within that thread, would be all of the juicy details that people who haven't seen the film wouldn't want to see. Going back to the Prometheus thread, I fully expect there to be a second thread opened on June 7th for story discussion for those that have seen the film.
There is a statute of limitations, though. Once a user has enough time to reasonably be expected to have seen the film, then it's fair game. And really, if you haven't seen the film within the first year of release, then you probably don't care enough about it to consider yourself spoiled for reading plot details.
Penny Arcade - As Regards Spoilification
For TV shows, I would just have a dedicated spoiler thread and be done with it.
