Here's an XNA engine...they claim to be the first full development system using it. $249 and $99 licensing:

http://www.blade3d.com/

I recognize that one model...I have it on my drive somewhere. Anyway, it looks like a BSP engine with native support for FBX and X and of course C#.

My User Contributions master list - my initial post links are down but scroll down page to find list to active links