As of right now, they are NOT on XM. XM carries Pac-10, Big 10 and ACC football and basketball games. Haven't heard if they'll be picking up the SEC anytime soon.
Trust me, they'd love to have the SEC . . . but it's not like they can just start broadcasting games. There's the small matter of the SEC granting them the rights and then negotiating a fee.
I've got XM. Once you get commercial free satellite radio where you can listen to exactly what you want when you want it, you realize just how much crap you are subjected to on regular radio.