HD Radio is working with Apple that will allow HD Radio listeners to tag songs for later purchase in iTunes. The service will works only on supported devices, while listening a song can be “tagged” to appear in iTunes so you can purchase it.
The service is a joint effort between Apple and iBiquity Digital. Broadcasters who participate will need to implement the system on their radio stations.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.

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The online music service smstunes.com already have this tagging service for most popular US based FM radio stations (no HD), you just need to send a text message with the station frequency, not as easily as pressing a button but still can be useful as you can use it anywhere in US with your mobile, more details here:
http://www.smstunes.com/radio.aspx