Comment 16 for bug 1158432

The issue seems to be that NetworkManager stores the connection credentials within the keyring, which is not available at the Unity greeter. Once logged in, the keyring is available to NetworkManager and hence nm-applet will not ask for credentials again.

I have found another workaround for the problem:

1) Log in to the desktop.
2) Click the NetworkManager applet.
3) Select "Edit Connections..."
4) Click on the connection you use.
5) Click "Edit...".
6) Click the "General" tab.
7) Uncheck the "All users may connect to this network" box.

This converts the connection to a user connection rather than a system connection, and hence prevents NetworkManager from discovering it at the greeter.