After upgrading my Kubuntu to Jaunty, I had problems with Internet access via Wi-Fi. Speed was many times less than before the upgrade (only then I use WPA2 encryption). This way solve the problem:
$ sudo aptitude install linux-image-2.6.28-11-generic
and reboot, choosing this kernel
P.S. in KDE4, NetworkManager replaced by plasma-widget-network-manager. If you can't connect to your wi-fi network with WPA2 encryption just install "linux-backports-modules-jaunty-generic" package.