Michael Hyatt of Thomas Nelson Publishers announced that they have redesigned their intranet to be a wiki: From Where I Sit: A Practical Tool for Collaboration.
The wiki was put in place because the intranet was static; you had to go through IT to get a change.
As James Dellow pointed out [Marching the intranet retreat], using a wiki may just be a cheap web content management system.
The flaw in the Thomas Nelson intranet was the inability of a user to contribute content. An intranet should be about viewing content contributed by users. Although an IT group may be really interesting, they should not be the gatekeeper of user content.
The Thomas Nelson intranet must have been very static for it to be replaceable by a wiki. I imagine it was like my firm's first generation intranet that consisted almost exclusively of html pages that you had to edit with FrontPage.
Our current intranet is now much more of a portal, pulling information from other systems and displaying it for the user through the web interface. The most popular tool on our intranet is our photobook application (which is true for most intranets). The photobook information pulls information from the HR database. I would hate to have to maintain the phone directory on a wiki.
Using only a wiki for your intranet prevents you from incorporating other systems into the intranet. You can link to another system that is acting as a portal, but that would propel you right out of the wiki.
We are currently redesigning our intranet, moving it from Sharepoint 2003 to Sharepoint 2007, and trying to figure out how to include wikis in the new design. DO we have one big wiki for legal content or do we have several wikis focused on different areas of legal practice and administrative areas.