Library copyright exception - what is "normal commercial exploitation" ?

Not a trick question ... just one in need of an answer.  This week I was teaching librarians who are embarking on digitization projects Wednesday about the little-known Sonny Bono exception for libraries.  Section 108 allows libraries, archives and nonprofit educational institutions to make copies of work in their last 20 years, so long as the works are not subject to "normal commercial exploitation" (17 USC 108h) (nor obtainable at a reasonable price, nor on an owner-filed notice per 108h).  What does that mean?

The good news is that a library copyright study group has been convened by the Library of Congress ... with none other than our own Peter Hirtle as a member.  If you have suggestions on how to define "normal commercial exploitation" (or other library copyright suggestions), add them here or send them directly to the group.