and that is what I have going on here at work. My computers are seperated by a MNF firewall from the rest of the office, and I use samba to authenticate myself to the rest of the windows network.
I have used samba for authentication here at my office, but not in the manor you are describing. I just wanted to say that I thought in expert mode with the samba man page printed out, I found webmin to be perfect in regards to setup.
I logged into my system as root, just to see if I could reproduce the problem, and sure enough...it was just as tvlad said it was. I'll poke around for a fix, but I just wanted to let you know you weren't the only one.
Oh! now i see what you mean, well... If you can see the sources in dialog box but they just won't update, try updating it from the command line using this command:
urpmi.update -a
or if you have broadband try this site out:
http://plf.zarb.org/~nanardon/urpmiweb.php
I do this:
/home user settings/configs
/opt software that I install after the mandrake install
/stuff this where my files go.
This is fine for a one user system, but I doubt this model would work anywhere else.