Using referers isn't necessary. http://en.wikipedia.org/w/index.php?title=Gil_Prescott&action=edit is different from http://en.wikipedia.org/wiki/Gil_Prescot. Red links point to the former, which is clearly a 200 OK. A "link from external site or typing in the url" would presumably go to the latter.
But the "Edit this page" link points to the &action=edit too, but that certainly shouldn't return any kind of error code, since it does exactly what it says on the tin.
I'm not sure that 404 is the correct response though. Wouldn't it be more correct for http://en.wikipedia.org/wiki/Gil_Prescot to return a temporary redirect to http://en.wikipedia.org/w/index.php?title=Gil_Prescott&action=edit?
That would be doing away with the "No page by exists by this title" page completely - do we want to do that?
The spec is really rather incomplete when it comes to dynamically generated pages. 200 OK isn't correct, but neither is 404 Not Found. What you'd really want is 2XX Dynamically Generated. 307 is technically correct, though.
Really, you need a new section for Web 2.0 - at the moment, there is very little support for user generated content (there's "Created" and "Accepted" (202 and 203 give or take)). We need a "content requested" code, or something.