Comment 6 for bug 543042

Moving to nautilus, as PolicyKit itself provides a way to know whether authentication was canceled or whether it failed. See POLKIT_ERROR_CANCELLED, which is the GError returned by polkit_authority_check_authorization_finish(),
 at http://hal.freedesktop.org/docs/polkit/polkit-1-polkiterror.html.

So the problem may be with Nautilus not checking that value, or with the backend that Nautilus calls not returning that value as a special case. In the first case, that's a paper cut, in the latter, it's more complex.