Comment 18 for bug 1388130

It would really be quite surprising for unity-greeter to have any effect on nm-applet without breaking a whole lot of other things. The one thing pointed out that looked the most promising were the D-Bus errors, and the "no agent available" message in syslog -- that's really where we should be looking.

Please, see that your system is fully up-to-date, and if the bug can be reproduced there, then let's try to remove the nvidia proprietary drivers and see if this still has any effect on wifi.

The agent NetworkManager is looking for is almost always nm-applet (as from the network-manager-applet source package), except in conditions like in Gnome Shell. Perhaps there are logs in .xsession-errors (or now .cache/upstart/gnome-session-Unity.log?) that could explain issues on the nm-applet side if NetworkManager can't connect to it?