This is a very significant development as I argued back in April 2010 that it would be. Support for ePub in iTunes U provides faculty with an excellent vehicle for disseminating instructional materials in the form of ePub documents. These can range in size from 300-400 page textbooks to much smaller documents such as handouts or even syllabi.
We should also expect to see an upsurge in interest in applications that facilitate the creation of ePub documents and there are already many options to choose from. Apple's pages application added support for ePub as reported here back in August 2010. Pages and the iWork suite of which it is a part will undoubtedly become must have software.