It's a record label's dream: fan hears song on radio, fan likes song, fan presses one button on radio receiver and buys song.

Whether or not fans will actually do this remains a big question, but at least the technology is finally rolling out to give them the opportunity. iBiquity Digital Corp. and Clear Channel, after months of preparations, have rolled out their "iTunes Tagging" hardware and software system that allows users to earmark songs they like via HD radio broadcasts for later purchase on iTunes.

The first radio receiver to feature such capability hits retail shelves today -- the Polk Audio I-Sonic Entertainment System 2, available in Apple stores and online. It will find its way to select Best Buy locations this June. Alpine, JBL and Sony have committed to develop similar devices, but no word on when those can be expected.

While certainly an important milestone, the table top Polk device is unlikely to spur a rash of iTunes tagging suddenly. Radio is most frequently played in people's cars, not tabletop boomboxes, so the real tipping point will come with the introduction of car stereos include the functionality.

Meanwhile, Clear Channel said it has now added iTunes tagging features to more than 340 HD radio stations nationwide. In order for the tagging feature introduced by the Polk device to work, the radio station must carry the associated metadata behind each song, allowing the device to "tag" it for later purchase. The feature is available only on HD radio broadcasts.