When is a spoiler a spoiler?

Earlier today I was perusing the True Blood message board on IMDb.com when somebody very nonchalantly revealed the identity of the serial killer -- ya' know, the big mystery and central premise of the show -- in the midst of a discussion that was in no way related to that topic. This was a HUGE reveal -- one that could potentially very negatively impact my enjoyment of a show I have really been getting into.

The justification was that the show is based on a book series and that the identity of the killer was revealed in the books a long time ago.

This got me thinking: When is something considered a spoiler? For example, after an episode airs, is it appropriate to post what happened without any spoiler warning? Or is it proper etiquette to provide a "buffer" zone? If so, how long is appropriate?

Along with that, if something is revealed in a preview or commercial (such as the return of a character), is that fair game? I would think so, however I do know people who avoid watching previews like the plague, because they want to go in completely fresh.

And what if something is based on a novel? Even if it's a relatively well known book, should you respect the fact that not everybody may have read it?

Then there are classic movies. After almost 30 years, is it okay to talk openly about Darth Vader being Luke Skywalker's dad? And now that nearly a decade has passed, do we need to continue to hide the twist ending of Sixth Sense?

I tend to take a pretty conservative stance. You'll notice that I only post the introduction to my television reviews (you have to click a link to read the full column), and I typically don't openly discuss shows like Lost without any spoiler warnings until after the season has ended.

Like I said, the identity of the serial killer could easily tamper with my enjoyment of the show. While likely unintentional, it was still an inconsiderate and unnecessary thing to say. But I am curious, at what point does everybody consider something "fair game"? And when is it still a spoiler?