In addition to the Tech Assistance section that's recently been added for Space Marine, the game now has its own dedicated modding forum... The Forgeworld. It's rather bare-bones right now, but NastyNoodle's texture thread and Santos' tool thread have been moved there at least. The rules for this section will be updated as needed to allow mod advertisement and discussion, but primary discussion of mods and modding should take place at The Forgeworld.