Q:

How to introduce versioning for questions on Stack*?

What today is the best answer for any given question, yesterday was not available and tomorrow will be obsolete. Especially when we're talking about software development.
Here is an example for you (there must be thousands, this one is absolutely imaginary):

Q: What is the best way to implement
autocomplete in javascript?
A: (2000) Whut?
A: (2007) Write a custom ajax function, display the results after processing 
A: (2011) Use this plugin: http://jqueryui.com/demos/autocomplete/ (nono, I'm not a jQuery affiliate, actually I prefer MooTools)

What would be your recommendation to introduce versioning for Stack Exchange questions and answers? Is there a need at all for that?
Edit after accepting an answer: Péter Török's answer has great insights. Indeed, there isn't probably a good way to introduce versioning for Q&A-s, as it would require semantic interpretation for each entry. Re-posting a question is the best way to go, I'd suggest to use the current date either within the question (What is the best issue tracking system for a small enterperise in 2011?), or as a tag.

A:

The problem is valid, however I am not sure versioning of questions / answers would help. In fact, in the example you show the question is the exact same, just the preferred answers change. However, very often an old question may not even make much sense anymore, or may be completely obsolete (e.g. "How to implement a typesafe enum in Java?" after the introduction of Java 5).
Moreover, the "best" answer is always selected by the current consensus. At any point in time, and for any language / platform / tool, there are early adopters, cautious adopters and conservatives. Usually the relative majority is in the middle group, so this is what tends to decide which answer gets the most upvotes on the StackExchange sites. And answers getting obsolete is caused by the shifting of opinions between these different groups.
But for a given thread (like your imaginary example) how do you decide that the answer voted as best in 2007 (and thus its upvotes and acceptance mark) is obsolete in 2011? Any new answer to the same question would start from 0 hence have a limited chance of getting to the top. So either you need an authority who decides what the state of the art is regarding a certain problem (which is totally against the community driven nature of SO), or you need a public re-evaluation of the question time and again, whenever the general public feels the old answers are not satisfying anymore.
For the moment, I can't think of any better way to this than reposting the same (or a similar) question as it naturally happens on SO. If the favourite answer(s) to the new question differ significantly from the old one, it is a sign that the two questions are not exact duplicates because the context has changed.
However, the current practice of merging duplicate questions may work against acknowledging such changes. The new answers can easily get buried in the group of older ones, especially if those got more upvotes back then. And if the new accepted answer significantly differs from the old one, what to do? In this case you may be right that somehow it would be benificial to keep the two questions - linked, but - separate, with some sort of mark that the answers to the newer question obsolete the older ones.

