the satcasters get your local networks via fiber, I would only guess and say yes that XM has a special link for them. sure would love to have a link in to that feed
By the way, Sirius channels (from E*) also sound much better than from a sat. radio. This is because sat. radio has a very low bandwidth - in my opionion only good for voice, not music (I heard somewhere that it is around 40 kilobits per sec!)...
So it's worse than FM radio, sounds basically just a little better than AM radio quality to me.