One of the biggest questions I get from people who want to use wiki.ucalgary.ca is "How do I protect or lock a page?" I've had to respond with a gentle suggestion that the wiki is an open resource by design, but that doesn't go very far. There are valid reasons for locking down a wiki - ranging from sensitive information that shouldn't be In The Wild, to protecting privacy (K-12 collaborations can't have info about kids leaking onto the 'net for obvious reasons).
It really felt counterproductive to just refer folks to other external tools like Writely. They're great tools, but if wiki is to be a valued resource on campus, it should serve all core requirements. I'm struggling with the possibility of migrating away from MediaWiki (which doesn't handle security, by design), or possibly adding a second "secure" wiki as an option.
I spent an hour or so this evening playing around with TikiWiki - some pretty fully-featured stuff (if a little "busy" as a result) that should solve any/all issues. I'll keep playing with Tiki to get a better feel for it.