Comment 2 for bug 351032

Yup, still happens here too, most often when resuming from hibernate.

Things that seem to help (or at least mean doesn't need a reboot to fix).
Don't use knetworkmanager use 'nm-applet' (killall knetworkmanager;nm-applet &) - only needed once at login
Restart network manager and/or networking (/etc/init.d/networking restart ; /etc/init.d/netwrok-manager restart) - this some times makes nm-applet re-ask for the wireless network password (even though it's a system connection and normally works fine) but just close the window.