All of the movies on the Xbox Live marketplace should support Dolby 5:1. I've downloaded two movies (Space Cowboy, which I didn't actually watch, and 300, which was awful.) but I only downloaded the SD versions. I personally wouldn't bother with them again as I found them to be a great waste of marketpoints. 300 looked fantastic on my tv even though it was only SD, so i'm guessing i'd look much better in HD.
But back to the point, if you are going to download them, I do believe they all support Dolby 5:1 as well as other types.