According to an article from the New York Post (I know, not my usual quality of journalistic source), many of the major artists who have so far refused to sign up for iTunes have done so because they want to be able to sell music in only album-sized chunks.
And, who can blame them? When you have one-hit records, wouldn't you rather sell them for $14.99 (or even $9.99) because of your need for the "artistic integrity" of your album, as opposed to selling a $0.99 track?
I'm disappointed, but not surprised if this information is true. However, based on my previous survey of certain popular artists and their lack of presence on any of the major services, it would not be a shock.
I, for one, hope Apple sticks to its guns. With any luck, the indie event last week will cause a surge in new releases on the iTunes Music Service over the next 90 days and we'll the success of those efforts will bring the reticent artists around.