According to music industry blog Coolfer, average weekly sales of Dark Side of the Moon nearly tripled following the launch of iTunes Plus. Other albums are said to be enjoying increased sales, too—sales of Coldplay's A Rush Of Blood To The Head and OK Go's Oh No have jumped 115% and 77%, respectively.
But interestingly, those increases in digital sales seem to be occurring directly at the expense of CD sales. Since the iTunes Plus launch, Coolfer says, Dark Side of the Moon CD sales dropped 12%, A Rush Of Blood To The Head CD sales dropped 24%, and Oh No CD sales went down 45%. Coolfer deducts that the introduction of high-quality DRM-free downloads "has somehow accelerated the CD-for-digital substitution that has eroded CD sales." (Thanks to Ars Technica for the links.)