Suppose I run into a popular question with a (potentially) outdated answer. Consider this one about Subversion.
I am (thankfully?) not a Subversion expert, so I don't know just how up-to-date this information is, but I know there have been several releases since this question was asked and answered. So I am not qualified to vote and say it is outdated nor to update or delete it (which would be a shame for folks on older versions who use this as reference anyway). If I re-asked the question, it would basically be a duplicate of the popular and well answered one -- but would otherwise be the most effective way to get eyes on the question and an up-to-date answer.
Suppose I knew an updated answer. I could add it to the bottom of the question, but it would take a long time to bubble up to the top, if it did at all, and on a popular question, weeding through all the low vote answers isn't always the best use of time (and undoubtedly spending time with the accepted and more popular answers which may not be up to date).
Virtually every question on SO has an implied modifier: "in the current version". How do I do a merge in Subversion (in the current version)? This might be crazy talk, but if we could tag answers with versions, then basically when a new version comes out the community could regression test the answers. If the answer stops being valid, the person interested in the answer could re-open the question and become the person who accepts a new answer.
In the case of the Subversion question, I could tag the top answer valid for subversion 1.3, 1.4 (totally an example, I don't know if it's true) and somehow mark "Is this the best answer for Subversion 1.6?"
Occasionally a question itself might only be valid for particular software versions, and that too could be marked.
This is half-baked, but perhaps someone has a cleaner idea that could work?