Comment 29 for bug 431147

@ Neal and David - Neither of you have polkit-gnome-authentication-agent-1 running in your session, which is required for authentication to work. This is most likely because you've unintentionally disabled it in System -> Preferences -> Startup Applications.

In any case, this is a local config issue rather than a bug, so I'm closing it.

FYI - we're hiding polkit-gnome-authentication-agent-1 from the session dialog in Lucid, to stop users from accidentally disabling it and having issues like this (see bug 509798)