This one works for me : http://pastebin.com/kDXqafxf
This is Leiche's apt-sources-update-0.3.1-4leiche2011.i586.rpm
version with a "ktsuss" added in front of "synaptic &" at line 133.
This version creates the directory repos-test-speed in the home user, prompts for root password before rewriting a sources.list, contains no special variable for "kdesu -c" or other "gksu -l" or $GSU, and does not even contain the new box for the bandwidth. It does work correctly until the end, and opens Synaptic and the repos list window. Then when I install new packages I get no error message related to ldconfig.
ktsuss is a small program that aims to keep the "su" simple, stupid, and free from bugs.
It is still maintained and a new version will be available soon for an update. (1.5 I believe). In my main machine it has also replaced "gksu -l" in the synaptic.desktop file : with success as all works as well with it.
Leiche, you have worked a lot making new version after new version, I have tried to keep the rate and test, for this one it is also the best I can do for now (I know you need to stop with that now, you have told me in your mail). I also need to go do other things. I hope theses tests and feedbacks will be useful in the future.