Hi, we would be needing the equivalent of "su -" for the actual Synaptic, which will otherwise produce a "ldconfig : command not found" error message for post-installs.
The ldconfig issue exist because using 'gksu' doesn't set the PATH to include "/sbin", and generally speaking the root's PATHs:
gksu -l include the root's PATH but also change the home and current directory.
gksu (without -l) doesn't include the root's PATH nor it change the home and current directory.
kdesu appear to set the root's PATH, but doesn't reset the current directory
My personal view is that the call to ldconfig should be "/sbin/ldconfig"... but this fix could be difficult to apply.
The most simple workaround in my view is that the apt-sources-update script work in a different place, i.e. /tmp, instead of user's HOME, of course, other workarounds are possible.