With more and more Illustrators working digitally, I can't help but believe the lower costs of printing, for the Publisher, will result in more PB sales, as the years go by. Where once a publisher would have to pay for shipping of original art, both from the artist then to the printer, then back to the artist again after use, the extra printer costs of scanning and/or using the old drum roller, colour separation, etc, the Illustrator has, inadvertently, replaced it all (at no extra cost by the way:( ) by sending digitally transferred, finished art.
Also, with the perfecting of interactive apps, colour readers (iPad) and the wonderfully high resolution available on these, to show case the Illustration, Publishers will be realizing more and different ways to earn from one PB.