The NVIDIA part is most likely due to missing shader capabilities on the ATI end
The ATI X2900 is a perfectly compliant DX10 card, and supported by all DX10 games too. Actually ATI only covers mid-end solutions, but compared by price and performance to nVidia very good ones nonetheless.
You can't narrow compatibility to a single brand (Konami once did with Metal Gear Solid 2 Substance, and the reaction was so harsh they had to add ATI support with a patch in a few weeks), and for an engine having to shave everything but Nvidia Series 8 and DX10 is plain silly, since a broad userbase is always the most important important aspect for a shareware/commercial developer. It's a lot easier to treat the pc as a console (limiting your hardware choices you don't have to look at scalability and compatibility) but the PC community is too mixed hardware wise... I could understand about going DX10 only (and I already argue on that, at least in 2007) but limiting the choice to nVidia only shows lack of committment on the developer's side.