I have the same problem as Network Manager Applet menu freezes and does not respond in ubuntu 12.04.
Now, even restarting the nm-applet won't help.
Is it a bug? I thought 12.04 would be pretty stable now.
What could I do then？
I think I am still having a similar bug on my 12.04 LTS system. For now, restarting nm-applet works for me, as described in this link.
Certainly this is not a fix, but it's a quick way to get nm-applet working again (for me). In fact, when I'm lazy, I can just start a new instance of nm-applet without killing the old one. Maybe someone call tell me why that's a dangerous idea, but for now it's helping!