Can you try, from a command line, running "gksudo synaptic" (rather than "sudo synaptic"). Gksudo is the GNOME dialog to give you sudo access--if something is wrong with that, then that will block any program that attempts to bring up that password dialog--producing exactly the results you see.
Ah, I think I might see where the problem is...
Can you try, from a command line, running "gksudo synaptic" (rather than "sudo synaptic"). Gksudo is the GNOME dialog to give you sudo access--if something is wrong with that, then that will block any program that attempts to bring up that password dialog--producing exactly the results you see.