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! Id Software's Willits: Rage Mods May Not Be Possible [Gamasutra]
Wow, how sad.
Games are getting more and more self-contained. It's not that unusual for a singleplayer game to not be moddable, but hey - this is id! They invented modding as we know it.
A comment on the hardware requirements, though:
Half-Life 1 took up to eight hours of render-time for finishing lighting/radiosity data on a custom-built map - and it didn't exactly hold people back from making maps.
This sounds kinda similar.