I was confusing myself too much without nailing down more concrete examples, so I took a shot at coming up with user tagging usecases and a way to implement them with a flat schema.
The usecases may be biased toward a flat schema since that's what I had in mind... so feel free to add more, or change the usecase names or descriptions to make more sense. http://wiki.apache.org/solr/UserTagDesign -Yonik
