Duh! But even then mediawiki.org does not produce really any direct hits. And DefaultSettings from 1.9.x doesn't contain it either. Google didn't help, the only hit it produces is pointing to an archive of this very thread. :/
http://www.google.de/search?q=wgProtectedNamespaces&ie=utf-8&oe=utf-...
Can't you simple give me the URL where I can read more instead of letting me chase around the web?
That's because I gave you the wrong name. It's $wgNamespaceProtection. I'm really sorry about sending you on a wild goose chase. Here's a URL that explains it:
http://www.mediawiki.org/wiki/Manual:Preventing_access#1.10_upwards