It's hard to overstate how important I think this is. There are a few reasons.

First, this brings a focused, clear path for educational content on the iPad. By opening the Books marketplace to more rich multimedia applications, Apple has made it possible for many of the dreams of interactive learning advocates to go mainstream. Imagine learning math by interacting with functions directly or doing symbolic manipulation directly. Or learning to code by actually doing it alongside a narrative. It's hard to imagine an area of study that would not benefit from creative use of technology in teaching it. (No, just adding videos and audio is not enough.)

Second, it opens the door to getting iPads in every classroom. By making it so there is a clear incentive for schools to buy iPads for all their students (cheaper, more useful textbooks) you get an iPad in the hands of every child. It goes without saying this is a big deal.

Finally, it opens the door for real competition for textbooks. By capping the price of textbooks at $15.00, students can easily decide to have several different treatments of the same topic on their iPad. If you manage to create content that is more clear, enjoyable, or even correct, parents and students will be one tap away from getting access to it, cheaply, if their officially-sanctioned book is not doing the job. This turns traditional textbook publishing on its head, because it empowers parents to overturn the textbook choices their school makes from the bottom up. If the entire math class has switched over to using an alternative textbook to learn a topic (just through word of mouth), this might tip the scales so the best content ends up being adopted by schools. Creative, upcoming authors will find that usurping the mainstream textbooks is now possible. I can imagine maverick publishers basically taking the same table of contents from a mainstream textbook and modernizing the treatment of the topic, so students have an easy way to move forward in their class at the same pace but with a much better learning tool.

And this is just what is on the top of my head, before forward thinking content creators have gotten their hands on this new platform.