Now that Koha uses Business::ISBN, the handcrafted code for generating a display version of an ISBN should no longer be necessary -- DisplayISBN can just be a wrapper for Business::ISBN.
Actually the only place where DisplayISBN is used is in C4/Biblio to retrieve a publisher code. Therefore the routine (which did generate some oddly formatted iscns!) could be removed if Biblio uses Business::ISBN directly
Actually The calling routine GetPublisherNameFromIsbn is not used either. Sent patch to remove both routines
This bug is mentioned in: Bug 5403 Remove DisplayISBN and GetPublisherNameFromIsbn http://lists.koha-community.org/pipermail/koha-patches/2010-December/013268.html
Pushed, please test NOTE this will break some unit tests, that is ok, I have purposefully done so so that I can use it for a teaching tool with my students tomorrow.