Steve himself has hinted at improvements "coming" soon to iBooks, and 9to5Mac claims to have nabbed some internal Apple documents that lay out those improvements. The biggest feature is called "Collections," and it sounds like it works the same as Folders in the rest of iOS -- you can combine items in iBooks however you like, and the collections you've got in iBooks can be viewed right on your bookshelf in the main screen. Sounds handy, if not all that revolutionary.
Second, PDF users will get some nice additions -- PDFs will be able to be shared via email as well as printed off with AirPrint directly from the iBooks app. That functionality won't work for actual book files, obviously, given that if you want to read one
