You may wish to look at the programming resource on my website as it features an .x object collision system. Also I gather there is a .dll now called coldfusion or something that also handles .x collision.
The answer to your question actually lies in the intersect object command. If you are using .x objects for your level design then it is a good idea to learn this command. You will see it in action in my programmers resource, and various poly collision functions such as Kensupen's excellent sliding collision demo that inspired me to write mine. I'm sure there's more in the codebase too by now as well.
Pneumatic Dryll
