Comment 16 for bug 82927

Did some more investigation on this.
Apparently, /etc/network/interfaces does not play a role with NM.

A better course of action is this:
1. Check /var/log/daemon.log, the location where NM logs its messages. Notice any messages about DBUS not running, as this is commonly the source of the problem.
2. Open a terminal window as current user and kill the "nm-applet" process. Start it again from command line, "nm-applet" and check if anything changed (probably did not).
3. Kill again "nm-applet" and run it with "sudo nm-applet".
Observe any messages on the command line from this command, and please post here. Hopefully by now it should work. If it does, kill NM again and run without "sudo". Check if it works now.

Good luck!