It's been more than five years since Apple's iTunes store changed music retail by introducing single-track digital downloads. But it wasn't until 2008 that most musicians and labels started to wonder out loud whether selling music by the track is good for their bottom lines.

Two of the year's biggest rock acts, AC/DC and Kid Rock, insist that their albums only be sold whole. And since Apple only rarely allows this, neither act allows iTunes to sell its albums in the United States.

Despite the fact that iTunes is the largest music retailer in the country, neither act seems to have suffered for this decision. As of press time, Kid Rock's "Rock N Roll Jesus" was the third-best-selling album of the year, according to Nielsen SoundScan. And it was sold only as a CD until almost a year after release, when Kid Rock granted Rhapsody the exclusive rights to sell it online in the States-as a full album. So far, the album has sold only 3,000 digital copies.

AC/DC released its new album "Black Ice" exclusively at Wal-Mart and has sold 1.6 million copies without any digital sales at all; it's the fourth-best-selling album of the year.

Both of these success stories challenged the accepted gospel that iTunes is an essential part of music retail. Perhaps more surprisingly, neither project...

Click here for the full story, which includes a look at digital hits from Lil Wayne and Coldplay, how the digital sales strategies differed, key points to consider when outlining a digital sales strategy, and more.