...Apparently this link led to website that claimed you needed to have a special .pak in order to play co-op. People downloaded it and joined this server this kid pasted. Upon joining, their game crashed and when they tried to log back in they got "CD key in use".

After multiple reboots and no change, it was concluded that this was a key theft tactic. The kid admitted to it a few minutes later but apparently with this .pak, and by joining a server that's not encrypted, the admin is able to get your CD key which is what he did. Sure didn't take long for the first "hack" to be released for this game. The unbelievable part of the entire thing was that the site that the hacked .pak could be found at was a REAL Doom 3 site. Foreign, of course, but had a domain and a beautiful layout. It looked professional basically.

--From a forum, haven't seen it anywhere else so can't verify.
forums.guru3d.com