Ian Dixon has a post with video about using the Windows Vista version of MCE through his XBox 360, Running Vista Media Center on my XBOX 360 (with Video):

Last night I played with the extender function of Windows Vista. This is the first time I have got my XBOX 360 working as an extender with Vista.
Using the latest build of Vista (5472) I setup up the XBOX 360 as an extender. The setup was very simple, it seems slicker that the XP version and maybe a little quicker.
Once up and running it looks exactly like Media Center running on Vista, with the new UI and sounds.