It appears that Sirius/XM is finally releasing an iPhone/iPod touch app this week, probably on the 18th. Subscribers who asked about the application are receiving the following email:
Thank you for contacting SIRIUS regarding whether or not there is a SIRIUS iPhone application available. We are here to help! We appreciate the time you have taken to contact us in regards to this matter! We are happy to inform you that beginning June 18th a SIRIUS Internet Radio application will be available at the Apple iTunes App-Store for the iPhone and the iPod Touch! All Premium SIRIUS Internet Radio subscribers will be able to download the application from the Apple iTunes App-Store for free on their device and access the service at no additional cost! Subscribers who are listening to the Basic Online Service for free must first upgrade to the Premium Online Service for $2.99 per month to access the application on the iPhone or iPod Touch.