Its this plastic decking stuff, that seems to be becoming the first choice for the pro's. When wet it slides really well, but burns like a motha' when you fall.
I didn't get a chance to see the games, but having a look on the WW X-games gallery, it seems to be PVC pipe. But someone who was there would best know that answer.
I guess that would work fine. I thought they got the decking from home depot type store. As Shawn said, Metal still works fine. I think PVC might be the cheapest.