Tony Karrer has an instructive posting about wikis. He writes about how often wikis are ill-considered as an elearning tool, primarily due to fears surrounding the insertion of errant or low-quality information by individuals who have little or no expertise. In fact, The Wired Campus has an opinion piece that makes very salient points about new learning technologies such as wikis, and the as-yet unquantified impact on academic leadership. Tony is correct to point out the crowdsourcing nature of wikis. I would also emphasize the power to correct that which is incorrect, a central feature of the wiki system. Some may argue that a post-counterpost struggle may ensue, with wiki modifiers possibly being at odds with wiki admins, and either party insisting they have the correct information. But Tony is again correct to point out that just as the information can be corrected, so too can the perspective and the information others have.
A while back I wasn't so sure of the wiki idea myself. But while at a friends house we were discussing Wikipedia on his big screen TV (computers displaying in 50 inches...greatness), and he demonstrated for me the correction system. He found an historical article, and made a change that made a particular statement incorrect. Within probably 30 seconds after posting and see the new statement in official form, it had been corrected back to the original form. This quality control is certainly susceptible to biases and malfeasance, but I have since become a big fan of Wikipedia, particularly as a means of locating sources for Masters papers I am writing. My university is dutifully wary of Wikipedia as a scholarly resource - which it of course is not - but at the very least I can link to some sources I had not yet found, or branch into related subjects, another power of the wiki format.
My most recent contracting project had me creating a wiki for the program team, partly as a means of communication regarding the program's aims and projects, and getting the word out to the wider workforce. This wiki was fast becoming a good resource on the company's intranet for locating material on the company's history, various industry topics, departments, and other types of knowledge. It would be hard to say that the wiki was becoming a tool for storing business intelligence, as much of the internal information was sensitive and likely unacceptable for company-wide posting. But as an intranet tool the wiki appeared to be serving its purpose well, and the ease of entering the wiki and making editions to information empowered all members of a project team (for example) to enter new information when it became relevant.
I'm still getting to know the wiki, but I do find the participatory element of wikis to be a good thing for engaging others in learning. As such, whether the wiki succeeds of fails as a central information resource on the internet, it seems likely to succeed as a key learning tool.