id say that user mods of their new driving/shooting things game Rage might not be possible due to the complex way the game engine handles textures.
The id Tech 5 engine uses 'MegaTextures' - huge texture maps (up to 128000x128000 pixels) that require huge amounts of preprocessing to create.
id - and John Carmack in particular - are still very much pro-modding and have confirmed their plans to eventually open-source id Tech 5, but are unsure if there is a way around this technical limitation. Simply put, it is unlikely that any modder would have the computing resources that are required.
Just a thought - this sounds like an area where a project like Folding @ Home could be useful. Sure, cancer might not get cured as quick but, c'mon - MegaTextures!