This is going to be unfortunate for Linux, and Mac and Windows users... it appears in the next month Apple is taking down some of the iTunesU collections - which will only be downloadable from the iPad or iPhone:
"If you want to be able to access iTunes U content on your Mac or PC after the move, download the materials to your computer before the move" :/
As a developer of Tunesviewer I am sad to see they're shutting this down... If you have been thinking of reviewing any courses on iTunesU now would be the time to download them!