Thousands of textbooks, including books from Cengage, Pearson, and McGraw-Hill, Worth, and Wiley are already available on the iPhone. The screen's a bit small, but the convenience factor is huge, and the books rent for 6 months for 1/2 the price of a paper copy. Check out a video here.
CourseSmart seems to be getting ready for the Apple Tablet. This could be very interesting, as soon as Fall 2010. Check out the video here.