GigaOM links to this article about user-generated tags.  The conclusion: That user-generated tags can be trusted.

The study uses a game in order to tag media.  The game awards points to players if their tags match.  The tags generated by the game were then compared to a series of dictionaries of tag words.

The total of verified tags was 41%, but given a different methodology of verifying the tags, the number could well be different.  My guess is that the validity of the tags would be considerably higher.

For example, here I tag my own blog posts, but I am certain that users could come up with valid tags that I simply don’t think of when I post my entries.

Systems that allow users to add tags and other metadata to content without requiring creators’ input would be useful to both the media creators and to other users of the content.  Furthermore, users can keep the creators honest by rejecting invalid tags.