Another idea would be to not allow anons to create new articles (editing existing articles would still be allowed). They would get a message after following a red link that invites them to create a user account so that they can create a new page
Rather than *forbidding* or *allowing* things, why not use a more gentle method of discouragement? Simply don't display red links to anonymous editors. They can still create articles if they really want to by going directly to the appropriate URL, and experienced anonymous contributors would no doubt do just that.
I think this would also improve the reading experience for those who have no intention of editing.
-Martin