I'm with Wonky on this one. Until they come up with a waterproof, bite-proof, drop-proof ipad, parents are likely to prefer spending on a board book rather than handing over their equipment to a sticky-fingered kid.
I've seen some of the trailers for picture books on the ipad (one was for Alice in Wonderland and it was amazing!) and there's no doubt that picture books will also be viewed this way by a proportion of children whose parents aren't bothered if something gets damaged, but it'll take a long time before this format actually affects in-your-hand picture book sales.
Plus parents are continually being told of the importance of reading aloud to children and while books on a screen may have a voiceover it's not the same as having a real, live mum or dad sitting next to you, giving you time, using his/her special voice to convey those words on the page. For example, my daughter loves to watch episodes of Charlie and Lola on my laptop but if she has the choice between that and sitting on my lap with me reading her a Charlie Lola book? The book (or me!) wins every time. That will change as she gets older but for very young kids parental attention is KING! Or queen. He he.
Comparing picture books to music tracks doesn't really work. Music is bought by all kinds of different people; picture books are bought by parents/family pretty much exclusively. The market is also different. No one would argue that listening to a track on your ipod would limit your intelligence (when compared to live music, say) whereas I bet you could find a lot of people who'd argue that putting a kid in front of an ipad picture book is less likely to get them engaged and learning as sitting with a family member/carer and reading the same story out loud.
Oops, seem to have prattled on for quite some time. I feel quite strongly about this!