The service works as follows: on supported devices, when listening to HD Radio, a song can be selected or tagged. Later, the song will appear in iTunes and can be purchased.
The service is a joint effort between Apple and iBiquity Digital. Broadcasters who participate will need to implement the system on their radio stations.
Right now, the user will need a compatible system, and the only two available in time for Christmas will be the Polk i-Sonic Entertainment System and the JBL iHD. Additional products are expected in 2008 for home and car.