While writing up some HD Beat posts on Window Media Center, I noticed that XM has a nice integration with the operating system. You don't gain any additional functionality over the standard on-line XM streaming, but the interface is a little cleaner to me. The interface takes up the entire WMCE screen, so you can resize until your heart desires.
I use WMCE to record and pause live high-def television; if the industry could work out any DRM and royalty issues, this interface would be great for recording satellite radio and streaming throughout your house. I do that with my HDTV signals; wouldn't it be great to do the same with my choice of digital music too?
