There's a thread in the SternFanNetwork forums about receiving Sirius XM on the iPhone, and forum member JJRousseau, who is apparently an employee of developer NiceMac, LLC, pointed everyone to the StarPlayr website. StarPlayr currently makes a Mac player and is hard at work on the iPhone app. JJRousseau noted in the forum that "[t]he iPhone is our first priority, but it has taken longer than expected getting licensing worked out for the iPhone Client. As soon as the business side of iPhone version is worked out, you'll be able to purchase StarPlayr for iPhone at the App Store."
The feature set sounds impressive, with Sirius and/or XM content streams, album art, iTunes WiFi purchase links, lyrics, and favorites lists. No release date or pricing has been set for the app at this time.
Thanks to Rymon for the tip!