Sirius XM just released their Android application and you can download it at siriusxm.com/android. You won't be able to listen to Howard Stern through this app because of a licensing rights issue, but you can listen to other Sirius stations while playing games and surfing the web. Ooh, multitasking.
If you're not a Sirius subscriber, you can test out the app with a free 7 day trial. After that, the service requires a subscription like the iPhone version. Most Android phones will be able to download Sirius XM except AT&T's Backflip, which doesn't allow apps to be installed outside of Android Market. [SiriusXM]