If the digital debuts of Led Zeppelin and others are any indication, the long-awaited debut of the Beatles on iTunes will boost sales for the Fab Four's music, regardless of the format.

Led Zeppelin
When Led Zeppelin went digital on Nov. 13, 2007, it was the same week that the "Mothership" double-disc greatest hits package was issued. So the hype around that title may have played a factor in driving the band's digital sales. In the first week of Led Zeppelin digital availability ending Nov. 18, 2007, individual tracks scans were about 300,000 units, according to Nielsen SoundScan. By the following week unit scans were 158,000 and by the third week of availability they were at 92,000. In total, Led Zeppelin's track scans through the end of 2007 totaled 981,000 units.

But beyond that, Led Zeppelin physical albums sales, which had been averaging about 8,000 units a week, almost doubled to about 15,000 units on average, selling nearly 77,000 units during the first five weeks of digital availability. Those totals excluded "Mothership" sales of 382,000 physical copies and 56,000 digital album units.

Interestingly, digital albums for the rest of the Led Zeppelin catalog didn't match CD sales for that time period. The launch week saw 13,000 digital albums scanned and 6,000 by second week. Digital album scans were at 4,000 units through the third and fourth week. By week five they were up to nearly 5,000.

Radiohead
On the other hand, if Radiohead is an indication of how the Beatles' CDs will fare, the Fab Four's digital release could result in cannibalization. Like Led Zeppelin, Radiohead's digital launch was also timed with the release of the "Best of Radiohead." On Nov. 3, 2008. On Radiohead digital launch week, track sales totaled slightly above 111,000 units, and a total of about 108,000 units in the four subsequent weeks.

In the four weeks preceding the release of that album "Best of Radiohead" -- both the single and double-disc versions -- and the launch of digital, the rest of the Radiohead catalog's CD sales averaged slightly above 4,000 units a week, for a total of nearly 18,000 units. But for the time period consisting of the launch week and the next four weeks, CD sales fell to slightly above 3,000 units a week, or a total of 16,000 units during that time period.

Meanwhile, the Radiohead album catalog scanned almost 15,000 units the first week of availability, of which about 5,000 was the "Best Of" package.

Before Radiohead's digital track launch, the bands albums were available on sites willing to sell the digital album in its entirety, like Rhapsody. Other sites like Napster and iTunes refused to carry them until digital tracks could be sold individually. Prior to allowing track downloads, Radiohead's digital album sales averaged less than 1,000 units a week.

Other Holdouts
With the Beatles going digital, the holdouts still include Bob Seger, Garth Brooks, Tool, King Crimson, Def Leppard, and to a lesser degree Kid Rock, who like Radiohead before him, will sell albums digitally but not allow individual tracks to be sold.