I know how the system works. Here is a brief descriptor from Wikipedia:
"There is an intentional four-second delay between the two satellite carrier signals. This enables the receiver to maintain a large buffer of the audio stream, which, along with forward error correction, helps keep the audio playing in the event that the signal is temporarily lost, such as when driving under an overpass or otherwise losing line-of-sight of any of the satellites or ground repeater stations."
You will note that they talk about the line of sight antenna-to-broadcast-source. I have experienced that the satellite frequencies DON'T penetrate buildings, or even cars, very well. And the cell phone in a walking around, or driving around pocket, will be variably shielded from line of sight to sources.
"Each receiver must be connected to an external antenna, which is included with the receiver. Antenna placement is crucial to receiving a clear signal. In some locations users have experienced difficulty receiving the Sirius programming because the signal is not consistently strong. For best reception, antennas should be placed such that they have an unobstructed view of the sky (preferably on rooftops without overhanging eaves or trees). If this is not an option, the antenna should be placed on an exterior wall."
I have technology reservations about trying to integrate a satellite receiver and antanna into a crowded iPhone. I have reservations that the loss of signal events would be frequent, even with the addition of a broadband component, and a 4 second delay to integrate the data streams.