Reuters recently published an article about the faltering prospects of terrestrial radio companies.
The terrestrial guys are in rough shape, ipods, SIRIUS and XM are picking off the most valuable listeners (from a demographics perspective) and are growing fast.
Another benefit to advertisers on SIRIUS and XM is that they can get an accurate count of who is listening. The terrestrial radio broadcasters can only use sampling techniques that can have significant bias in order to estimate their audience.
Google has led the way in making the results of advertising more transparent. I believe that will have a ripple-effect through the rest of the advertising world as advertisers learn to expect accountability and no longer satisfied with wishy-washy estimates of audience participation.