Big music stores squelch download plan | CNET News.com: "The first few weeks of Apple's store, in which millions of songs were sold to the relatively tiny audience of Macintosh computer users, had created a heady optimism among the retailers. When Apple moved into the Windows market, sales increased, but not nearly to the extent that the retailers had hoped."
IMO, the dirty little secret of Apple's success with the iTunes Music Store is that Mac users represent a disproportionate percentage of sales as compared to market share (which is a bad measurement anyway, but that's another blog entry.) I would bet that Mac users are responsible for at least 30 to 50 percent of ongoing sales through iTunes. I just don't think that Windows users are as willing to pay for content. Otherwise, wouldn't some of the WMA-based services have taken off in a big way by now?
I'm guessing Apple doesn't mention this secret because a. they don't want to offend Windows users, b. they don't particularly want to 'tip off' the competition, and c. it may not be possible for a third party to fully realize increased sales in the Mac market, simply because of the 'Apple factor' - that is, the third party wouldn't be Apple.