Interesting that imminent release of XM has brought about technology changes for the existing AM & FM formats. To wit:

A new technology will allow radio stations to supplement analog signals
with digital transmissions by the end of this year. This will allow AM
stations to achieve the same sound quality as FM, and FM stations will
approach CD clarity. iBiquity Digital CEO Bob Struble unveiled the digital
radio technology, saying "Static, hiss and pops all go away...You're
talking about a diametrical upgrade in audio quality." Radio World news
editor Leslie Stimson thinks the new technology, "...could revitalize AM
radio. A lot of AM stations have been forced into adopting talk formats."
FM stations will begin using the iBiquity technology in November. AM
stations will use it during the daytime until tests have confirmed that
night use will not cause interference.

http://www.techknowtimes.com/jump/v5i15016.html