According to this article on CNet News.com some recent events and discoveries around and inside the Wikipedia have forced the founder Jimmy Wales to change some of the basic handling flow of the whole engine. Until now it was possible to create and edit all articles anonymously. But Wikipedia is now big enough and so commonly known and accepted that the personal thoughts and impressions which sometimes are hard to keep out are the cause for conflicts with almost political influence.
So it was decided to disallow posting and editing for anonymous users although most changes and edits already originate from registered people.
I personally think that this is a bit a sad step but a necessary one.