What is happening with HDTV will also ultimately happen to radio as well.
XM and Sirius satellite radio are the vanguards of digital radio today much as DirecTV and Dish Network were to digital television in the mid to late 90's. Ultimately our existing AM and FM radio stations will also convert over to digital formats allowing us many more channels and much greater access to our media.
It may even be that WiFi (802.11.x) internet access may become common place across the world with little routers on every lamp post so that we can both stream Air Radio off the internet in our cars even as we are driving and talk via VOIP thereby offering new competition to today's cell phone systems.
Ultimately digital will help take radio and TV out of the hands of the Sean Hannitys and Rush Limbaughs and Bill O'Reillys of the world and offer a much more balanced and more diverse radio and TV world.