If it is, in fact, more effective at teaching students, it could potentially lead to larger class sizes, which could easily pay for the device. I am skeptical that it will be effective, but it could be - that's why they do pilot programs. I'm sure they will quickly begin to use it for other courses, too. I can see it being particularly useful for foreign languages (where being able to hear the text is important). I think the interactive textbook idea can also be really useful in science (especially at the early levels of biology, chemistry, and physics, where animations are so useful), geography (being able to pinch and zoom thousands of old maps, and being able to play games to reinforce learning), civics (being able to actually pull up the essential documents immediately). In fact, it's hard for me to think of a class that couldn't benefit from an iPad.
I think this will really turn out to be best for the students who want to move faster than the class and/or learn the material more thoroughly than required. For those students, this could help counter-balance the modern trend to dumb down the curriculum to produce high standardized test scores, rather than deeply examining the subject and teaching students to appreciate learning and thought. For most students, though, I expect this to be an overpriced toy with little educational value.