What I find really annoying about wikis in the government is that people think we need more than one. Having one wiki for everything in the government is the best way to get value out of this technology.
This principle is not unknown, yet time and time again people choose instead to create a wiki for a small group of 10 to 30 people. It is shortsited to limit the wiki contributers to a small group. The most value can be attained from a wiki when you have a crowd contributing. See Crowd Sourcing
If I were to do a statistical survey to find the size that this crowd would need to be with respect to your organization, I would espect to find that you need a 50 to 1 ratio between creators and consumers. So when I go to a page that I considered to be a quality wiki page, I would estimate that it has had around 50 people contribute to it for me then to be able to find value and become well informed. I would also expect that after about 50 contributers the quality will be improved but not as significantly.
If you want 50 people to actually get on there and make an edit, then you are likely going to need to market that request out to 1000 to 5000 people. Most Governments have those kind of numbers and could certainly make this happen but it needs to be marketed and sold just like anything else in life. And we can’t expect them that have to keep contributing to their small closed off group wikis.
To learn more about this a good article to review is Mass Collaboration on wikipedia