Moodle has had a wiki module for a while... and a wiki courseformat for quite a while too. It's in contrib, and I've encouraged its use often.
The idea is good, and yet the teacher has to be ready for it, so it's good to offer it as an option... which is what Moodle does.
(It'd be better if it was part of the official moodle release, naturally, and I hope the Moodle 2 wiki push will see this happen too.)
And as others have mentioned, roles let you do a ton of this. Fiddly (=improvable) but it works.