Yes, I've added support for non-GeForce cards (see my earlier thread
here), as long as the card supports EXT_texture_env_combine and ARB_multitexture (includes nVidia cards back to TNT, and ATI cards back to Rage 128, I think). It works well with games that don't use overly complex combine modes, but with those that do (like Zelda), it doesn't look as good as on a GeForce. I'll try to post some shots of EXT_texture_env_combine mode tomorrow so you can see what it's like for yourself.
I'm not doing pixel shaders right now because I only have a GeForce 1, but when (if) I do get a card that supports pixel shaders I'll definately add a mode for it.
BTW, does anyone know how many texture environments the Radeon supports? I might be able to improve it by using more than 2 texture environments.
Do I have plans for a Linux port? Not at the moment, but I'm planning on opening the source soon (maybe even next release), so someone else can do it if they feel like it.
I'll look into the 1080 sky, I still have several unimplemented combine modes in that one, so it might just be that. Thanks for pointing that out Rice.