The New York Times has an article in today's paper, “Rewriting History: Snared in the Web of a Wikipedia Liar” (free registration required), which tells the story of how an anonymous editor implicated John Seigenthaler, a former editor of the Tennessean in Nashville, with the murders of John F. Kennedy and his brother, Bobby.
The article does a good job explaining Wikipedia's weak point: anyone can maliciously create or update an entry in the online encyclopedia, and there are no checks-and-balances, no peer review before mis-information is accessible by anyone who happens to come across a bad entry.
The article does achieve some balance, however, by discussing some of the steps Wikipedia plans to take to make it harder for malicious editing. They also point out that once discovered, errors can be corrected quicker on Wikipedia than other encyclopedias.
As I've said before, the best practice is to have more than one source to back up your work. Don't just take Wikipedia as the gospel; find other sources to verify the information you find. That should be a practice not only for web-based information, but for all research.