Even digging through mud can sometimes lead to a hidden treasure. Now, I don't want to imply that StackOverflow is an equivalent of mud (though the fact is, most of the questions asked there are uninspired, to say the least), nor that this question related to REST APIs versioning is treasure - it isn't. In fact, it was so terse that it might have been overlooked easily.
Luckily, this wasn't the case. This answer, although over 2 years old now, is still an interesting read for those interested in RESTful services. I agree with the author - older API / resource versions should be encoded as part of the URI. However, in most cases, the URIs containing version number should not be permanent and should be phased out after some time, preferably redirecting clients to the current version (unless the resource is intentionally versioned, like for example a Wiki page). Also, there should exist a "canonical" permalink versionless URI for resources, that should always serve the most recent version
Obvious? Perhaps, but as can be seen in various answers posted to that question, the opinion is divided between people preferring to keep version as part of the URI and those advocating to keep it as part of HTTP headers (and one answer stating that you should never, ever provide access to previous versions of your APIs / resources - but let's not fool ourselves, this is not realistic in the real world). Ask an IT guy a yes/no question and you will get three conflicting answers, eh?