Solicitous is right that Kobo is bound to deliver the source code for GPL licensed applications (and LGPL licensed libraries) upon request. We have no requirement per the GPL to proactively make it available.
However, rather than have all of you email me and ask for the code individually, we've posted the source code to the applications in question, patches to the original code that we have modified, as well as the code to a special secret that you may have found in the Kobo Reader already to our github account.
You can find it here:
If a moderator is reading this, I would appreciate a change in the subject line. Kobo takes our GPL commitments very seriously, as you have hopefully seen via our full disclosure of what open source software we are using as well as our posting of this code.