Tharaldsen responded to Tracey's query with several pictures taken at the session, and a few quick words about the individuals who did it for them. It seems, somewhere in England, there is an entire stable and stadium set up just for this sort of work. The submitted images also show the mo-cap transition process, moving the data to a final product state. If you haven't seen pictures of horses taped up with ping pong balls today, make sure to check it out. Then read on below the cut for a demo video of mounted combat, taken at this year's GDC event.
I know I can't really think of a better way to do it off the top of my head, but when he gets off the horse and it blinks out of existence, that just kinda... there's a little WTF to that. I'm sure WoW players don't really notice that stuff anymore, though.