D pointed out this Boing Boing article describing a Nature article that examined (sort of) the accuracy of the Encyclopedia Britannica and the Wikipedia:
The exercise revealed numerous errors in both encyclopaedias, but among 42 entries tested, the difference in accuracy was not particularly great: the average science entry in Wikipedia contained around four inaccuracies; Britannica, about three...
Only eight serious errors [in 42 articles], such as misinterpretations of important concepts, were detected in the pairs of articles reviewed, four from each encyclopaedia. But reviewers also found many factual errors, omissions or misleading statements: 162 and 123 in Wikipedia and Britannica, respectively.
The upshot is that the study didn't find a large qualitative difference between the presumably authoritative Britannica and the presumably hacky Wikipedia. But remember this: neither source intends to be the end of the research trail. An encyclopedia entry is a brief summary of a topic -- there will always be omissions. These should not be deemed errors. What I'd really like to know, though, is whether Nature's reviewers passed on the errors to the publishers? And, if so, whether the errors were corrected?
You should still feel comfortable going to the Wikipedia for general information on general topics. For everything else, there's Three Way News.