I found this: http://xmfan.com/viewtopic.php?t=103751

The repeaters are for areas that would have problems with the satellite signals - congested cities, where the tall buildings will block the signals from (XM's) the southern-orbiting satellites. (XM's satellites are geostationary above the equator, approximately in line with Dallas and Atlanta, while Sirius's do a big figure-8 above the northern hemisphere.)

So, in your car, especially in rural areas, you're just listening to the signal straight from the birds. In cities, you could be listening to either the terrestrial repeaters, or the satellites.

The FCC has required that some of them shut down because their signal was too powerful and interfered with surrounding frequency bands. And, it appears that they've started that shut-down procedure. I'll find out more on Monday for y'all.