Nature responds to britannica (with a PDF, unfortunately).

In one instance Britannica alleges that we provided a reviewer with material that was not from the Britannica website. We have checked and are confident that this was not the case.

Britannica objects that Nature did not check the assertions of its reviewers. This is true; nor did we claim to. We realised that in some cases our reviewers’ criticisms would be open to debate, and in some cases might be wrong. But this applied as much to criticisms of Wikipedia as of Encyclopaedia Britannica. Because the reviewers were blind to the source of the material they were evaluating, and material from both sources was treated the same way, there is absolutely no reason to think that any errors they made would have systematically altered the results of our inquiry.

We note that Britannica has taken issue with less than half the points our reviewers raised. Both encyclopaedias have made corrections to some of the relevant entries since our article was published.

We do not intend to retract our article.