I used the WP-Calais auto-tagger on a blog that had about 2,000 posts. It worked wonderfully adding relevant tags.
The problem is, it generated around 19,000 tags, and there are only about 200 tags that are used on more than one post.
Could there be an option to delete tags all the orphan tags that are used only once? Or is there a simple SQL command that could do this? I poked around in wp_terms and didn't see a field for how many times a tag is used.