Gentlemen, it is you who are ruining network standards.
HEAD http://en.wikipedia.org/wiki/Some_Non_Existent_Page --> 200 OK
It is clearly a case of 404 Not Found.
You can still give the same "You can create this article" message AND return a truthful HTTP code.
Else how is one to use a linkchecker to your links? Why are MediaWiki wikis special?
Yes, 200 OK for action=edit, and disambiguation pages, but not for the basic clear case of the spirit of 404 Not Found.
What if all ==External links== always returned 200? How could a bot detect linkrot? Do unto others...