I keep meaning to link to this, but Katie Cunningham posted a good article on her blog about corporate wikis and how to keep them healthy. Definitely worth a read.
I agree with most of what she's written, although I'm conflicted about not putting everything in the wiki and having multiple wikis within a company. I can understand that having meeting minutes from five years previous might be considered cruft, but in my experience a lot of times that wiki page is the only place you're going to find whatever documentation might be left for that long abandoned project.
The problem with wiki balkanization is, again, that it makes it hard to find and share information within the company. The best solution may be to have different wikis for different organizations married to a powerful and effective search engine. But I haven't seen too many companies with powerful and efficient intranet search engines.
Aside from those nitpicks, I generally agree with the spirit of what she's saying and suggest any knowledge workers out there take a look.