Hey,
I just came across http://www.britannica.com/eb/article-9433599, which is the lead section of the Encyclopaedia Britannica article about wikipedia. There is also one about wikis in general: http://www.britannica.com/eb/article-9404276.
These articles appear to be taken from the current revision of the EB itself, not the yearbook or any other of these sources (on this web site, you can never be sure).
EB is now the second (or third, depending on how you count) encyclopedia to mention wikipedia. The Columbia Encyclopedia has one and the 21st edition of the Brockhaus encyclopedia (in German).
I don't have full access to the EB article from here and I am very curious about its accuracy :)
Fun fact: Last time I checked, Brockhaus refused to call Wikipedia an encyclopedia.
Mathias