The frustrating thing is the AAP is really not a good indication of overall ebook sales. For instance it does not show all the books sold by indies and small presses - a number which I think is significant. Since most indies and small presses sell many more digital books then print books it would (IMO) have a dramatic shift on the actual %'s.
I really wish Amazon and B&N would stop being so secretive with their numbers as this would tell us so much more than reporting publisher data and would give us some much more accurate data. Oh well...I'm not holding my breath for that one.
The source of the data comes from here. If anyone knows of other sources of ebook penetration I'd love to get a link or two.