Apple is not terribly specific about the issue, but it may be related to a complaint shared by USA Today almost immediately after the iBooks 2.0 launch and for which Apple had promised a fix.
We're aware of a small number of iBooks 2 users having issues with the playback of the introduction movies when opening iBooks textbooks. It will be fixed soon in an upcoming software update.
iBooks 2.0.1 is of course a free update to the already-free iBooks applications. A handful of iBooks Textbooks have launched, with most priced at Apple's maximum allowable price of $14.99.