CIO Insight has the shortest, sweetest guide to wikis behind the firewall that I've yet seen:

• Wikis are a social innovation, not a technological one.
• Wikis turn the notion of "permissions" built into traditional knowledge management DMS's and CMS's upside down; suddenly everyone has permission (and ability) to edit everything.
• If you're worried about people on your payroll vandalizing an internal wiki, you have a bigger and different problem; are people vandalizing the Coke machine?
• Last and of breathtakingly paramount importance:  "You can't know a priori what people need to know and share, but big knowledge management systems make a lot of a priori assumptions. Wikis don't."

I rest my case.