ATI takes a very different approach with their Linux drivers than nVidia. ATI provides their chip specs to driver developers who actually do the work of developing the drivers, resulting in nicely Open Source drivers that don't have complete functionality, while nVidia develops their own Linux drivers which provide a lot of functionality but are closed source.
I think there are features of their chips that ATI can't or won't make public for intellectual property reasons. I think S3TC support might fall in this category, because S3 required a separate license for non-DirectX implementations.