PRWeek has an article about PR firms adopting wikis. Good intro:

The flux of new technologies entering the market is enough to give the average PR pro a migraine. Just when one understands blogs, early adopter colleagues point out real simple syndication (RSS). Now water-cooler talk revolves around wikis, which can provide PR firms with internal communications enhancements; make writing press releases and communiques more of a team effort; and provide new opportunities for soft launches, tipping off journalists, and communicating with clients.

The local PR professionals I deal with fall into the category of the migraine masses. they are just now beginning to understand how blogs work and how to utilize them (if at all).

When I show individuals public wikis such as Wikipedia they all have the same response. Fear. Many can't understand how such an open system on the internet can function without falling into utter chaos.

Internal wikis are a different story.

The article goes on to list signs you might want to consider a wiki

• A press release went out with an error because the deadline was approaching and a change wasn't implemented
  
• Your clients look for you to be an early adopter of networking and social technology coming through the pipeline
  
• You often have projects that require the constant feedback of multiple users in different countries
  
• You've deleted the 20th office-wide e-mail by noon, and not one has had information pertaining to you
  
• Your intranet is too bulky and expensive for your needs
  

The last point is what I see as a major tipping point. Many organization have attempted to implement solutions such as MS Sharepoint or another 'intranet' solution. Most of these are far too complex for the average user, and are under-utilized. The wiki, like the blog, is such a simple interface, with a small learning curve that adoption by the masses is rather quick.

Need some more info? Common Craft has a great entry: Wikis Described in Plain English