1. For a suggest feature in search?

2. For tags display of content?

3. Improving the auto-generated tag system through validation, in an attempt to not completely rely on user generated tags?

There is a good thread to read on flickr about machine-tags that you can read here ~ http://www.flickr.com/groups/api/discuss/72157594497877875/

Using rdf you can represent tags as "foo:bar=foobar". So your use is really only limited by the limitations of the format itself.