Sat Rad Player is the first Windows Phone 7 application that allows you to listen to your Sirius/XM Satellite radio subscription on your Windows Phone 7 device. The app can sort Channels by Channel Name or Channel Number and also optionally group Channels by Genres.
Unfortunately not all channels are present, but this is more the fault of Sirius than the app.
The app will only work for users whose subscription includes the option for Internet play.The trial version limits continuous listening to 2 minutes, and will not allow for playing under a lock screen.
Sat Rad Player is a US-only app, costs $0.99 and can be found in Marketplace here.