For games, like movies or books, one of the worst things that can happen to you is that they spoil details of the story or even the finale.
How do we behave with questions/answers that reveal spoilers?
Why not do similar to what TVTropes does, and have markup that will change the text to the same color as the background, with a dotted box around it? The user then highlights over the text to see what was hidden. This will allow for in-line spoilers.
(If you haven't used the site, try here for an example; look at the bulleted point for "Strong Bad's Cool Game", 4th bullet from last)
Another vote for spoiler tags... adding them wouldn't necessarily even involve mucking with Markdown, though. How about a JS-based solution like the one used by Jay is Games?
We should relax about spoilers. Reader beware! Spoier is about providing unasked for (and usually out of context of the conversation) information. How can answering a question on Q&A site be spoiling?
Most people will arrive to the site (via Google) looking for a spoiler. Those who are afraid of spoilers will not come. Those in between should go to excellent http://www.game-hints.com/
Avoiding spoilers is just too much work. Look at this excellent question (and answer). Did you look at the title and go: "Oh, the warden is going to be caged?" How do you avoid a spoiler in such a title w/o ruining the question?
Even if we had a spoiler formatting option, using it may be bad idea. Most forums have it, but most users have no idea how to use it, and never do. StackExchange tries to make contributing as easy as possible, spoiler formatting option erects an unnecessary barrier.
Whatever clever system we invent will probably be lost on newcomers, and they are always in majority. Do we want to spend our time fixing their mistakes? Do you care enough to insert thousands of spoiler tags for them? Because if we make it a policy, someone has to do that tedious work.