I am experiencing a similar problem. I have network-manager-kde installed too and when I try to connect to a wireless network, it simply fails. When I tried to connect to an open wireless network, it configured the card right (iwconfig showed all the right things) but it simply didnt connect fully. It got stuck at 28% or something and said "Configuring devices". After it tried again multiple times and eventually failed, all I had to do was type "dhclient wlan0" in a terminal and it connected.
