Update: KDE 3.5.2

KDE 3.5.2 is now available for Kubuntu!
To update your KDE, type into your Konsole (one line after the other)

wget http://people.ubuntu.com/~jriddell/kubuntu-packages-jriddell-key.gpg
sudo apt-key add kubuntu-packages-jriddell-key.gpg
sudo kate /etc/apt/sources.list

Then add just one of the following lines to your sources.list (make sure the format is like this: deb URL in one line):

deb http://kubuntu.org/packages/kde352 breezy main
deb ftp://bolugftp.uni-bonn.de/pub/kde/stable/3.5.2/kubuntu breezy main
deb http://www.mirrorservice.org/sites/ftp.kde.org/pub/kde/stable/3.5.2/kubuntu breezy main
deb http://mirror.cc.columbia.edu/pub/software/kde/stable/3.5.2/kubuntu breezy main

In the konsole again:

sudo apt-get update
sudo apt-get upgrade

Depending on your internet connection it may take a little time :-)

You can more information on kubuntu.org and kde.org