Games Workshop has an IP Policy which explains what you can and cannot do with Games Workshop's Intellectual Property (IP). You can read this policy at
http://legal.games-workshop.com. We are in the process of updating this policy to make it clear we do not give permission for mods.
As you may be aware, Games Workshop has recently announced it has entered a licensing agreement with The Creative Assembly to produce games in the Warhammer setting. Games Workshop does not give permission for mods to be made using its intellectual property.