uname -a: Linux 3.2.0-1-amd64 #1 SMP Fri Feb 17 05:17:36 UTC 2012 x86_64 GNU/Linux Distro: Debian Testing. When attempting to tie <Shift><Super>comma to quodlibet --previous, I found that I couldnt trigger the command with the shortcut. XFCE was recording <Shift><Super>less as the shortcut - there is no dedicated less key, you must press shift and comma. Going with this idea, in xconf I edited the shortcut entry to <Shift><Super>comma and it worked. This is also a problem for > and likely other special characters.
Marking as duplicate of bug #8744 which has a patch for that. *** This bug has been marked as a duplicate of bug 8744 ***