Wikis are indeed a powerful collaboration tool for employees. However, like any tool that requires participation from people, the success of an employee wiki requires highly engaged and participatory individuals. In other words, a successful wiki has less to do with technology, and more to do with communications and change management.
Janssen-Cilag recently replaced their intranet with a wiki. The Australian pharmaceutical subsidiary of Johnson & Johnson originally had a static HTML intranet that was delivering little value. The original intranet (launched in 2001) was static, delivering limited value, and was plagued by many of the common problems associated with many young intranets.