By contrast, when a copyright owner sends a DMCA notice on a particular video, YouTube displays a message that says, "This video is no longer available because of a copyright claim by [the copyright owner]." Here's an example:
Again, I can't say for sure whether that's what happened here -- it's impossible to see who the account holder actually is, or what other videos he or she had posted to this account. But I'm pretty confident that this incident was not the result of someone at Rick Astley's label or the song's publisher suddenly deciding to rid the world of the RickRoll.
Update: YouTube says it was all a mistake. From a statement it gave to Mashable:
With 20 hours of video uploaded every minute to YouTube, we count on our community members to know our Community Guidelines and to flag content they believe violates them. We review all flagged content quickly, and if we find that a video does violate the guidelines, we remove it, on average in under an hour. We also have a team that is dedicated to identifying and removing spam from YouTube. Occasionally, an account flagged by users or identified by our spam team is mistakenly taken down. When this is brought to our attention, we move quickly to take appropriate action, including restoring videos that had been mistakenly removed and channels that have been mistakenly suspended.