I probably should have responded on the earlier thread - I didn't want to overwhelm the discussion. Anyway, I run the MediaWiki-based wiki farm Referata (
referata.com), and I'm actually planning to release the software that powers it as open source in the next few months. It's composed of two main extensions: Site Settings and Wiki Garden; the latter requires the former. I don't mean to try to discourage any other attempts to create wiki farm functionality - and anyway, more alternatives are always better - but I wanted to note that plan.