How Wikis Enable Enterprise Collaboration
The Five Keys to Enterprise-Grade Security and Infrastructure for On-Demand Wikis
Seven Wiki Essentials: The Must-Have Elements of Every Successful Wiki Initiative
Some very nice whitepapers discussing not only pbwiki and hosted systems but also why wikis are useful in an organization. There are also some very helpful case studies. Some people like to host wikis in-house but this requires a much larger measure of support and attention than a hosted site.
Many of these hosted sites have a lot of security measures to protect the integrity of the data. They also handle backups and hardware. They can often make a big difference with small companies. Particularly those that can not easily provide the time to support an in-house option.