First, although it seems that Wikipedia articles are produced by individuals without specific qualifications, most are, in fact, produced by a community of interested individuals. If there is enough interest in the topic, the article is reviewed by many eyes and is reviewed in real time in response to new research and changing circumstances.
Second, the real power of Wikipedia is on the discussion page of the article. In traditional encyclopedias, you never get to see the workings of the editorial board playing out. I looked at the article "Biofuel", since controversy is raging over the topic. On the discussion page, the biofuel wikipedia community is debating various aspects of the article and how best to present the information. The debates are completely transparent. And, you can really get more information about a subject from the debates that are going on around it.
Finally, Weinberger reports on a study by the journal Nature (Nature 438 (7070) p900-901) concerning the accuracy of Wikipedia versus Encyclopedia Britannica. The study found the accuracy to be roughly equivalent (although Britannica was found to be better written.) Although more studies will need to be done to prove or refute this point, initial results are encouraging. They also point out that even the venerable Britannica has inaccuracies.
Ultimately, the individual seeking the information is the final arbiter of what is true. Weinberger says
Deciding what to believe is now our burden. It alwasy was, but in the paper-order world where publishing was so expensive that we needed people to be filterers, it was easier to think our passivity was an inevitable part of learning; we thought that knowledge just worked that way.Before the internet, we assumed (to our detriment) that whatever is in print is true. This was never a safe assumption and now with the plethora of information available to us, we can no longer act in this manner. This is a call to get off our duffs and take responsibility for finding out what is true. And, it seems to me that one of the best indications of truth is the debates on the discussion page of Wikipedia articles--can the information hold up to community scrutiny.