Suing for libel is one of the sillier ideas to come up in this thread. If that were to happen Mrs. Doran would have a much stronger right of action than WMF.
The most important question about such a lawsuit is, "What would you hope to accomplish with it?" There would be a lot of legal costs to bring the matter to court. Would The Register even be able to cover those costs if we won? And think of the fun that all of Wikipedia's detractors would have while all the dirty laundry was being displayed in court and in front of the press. Maybe a court would come out with a declaration that we are right; that would still be an expensive declaration.
Forcing The Register into bankruptcy sounds like an accomplishment to me...