I've been researching this for a while. The problem is, sattelite radios (XM and Sirius) have receivers that pick up specific signals based on their sattelites. I have found a product that Sirius has been promising the market but as of yet, I'm thinking it still hasn't been released, but I haven't confirmed with Sirius yet. I'll do that this week I think. The product allows you to use your existing XM radio but install a converter to receive Sirius. Here is a link to a reseller who still claims it isn't released yet.
GM Sirius Connect
All this waiting for the car and the radio. Argh!