Just found this excellent paper by Laurence Parry from the 2006 Wikimania conference. It focuses on starting and building a community wiki, and a lot of his ideas, tips, and advice also apply to wiki use in organizations:

…why would you want to build such a wiki in the first place, and why would such wikis be successful? The answers are simple: the wiki provides (in most cases) a persistent storage for community history, while at the same time the community provides a ready pool of eager, knowledgeable contributors for the wiki.

Like the community Parry talks about, your organization has a pool of knowledgeable contributors whose knowledge should be captured on a wiki, both for their collaborative benefit, and for the greater benefit of capturing your organization’s institutional knowledge. Some people may be more eager than others, so it’s a matter of demonstrating the wiki’s value and encouraging them to participate.