I run Kubuntu 10.10, but you cannot update the KDE desktop to the newest version directly. There is a technique, though, that does allow the newest version of KDE—4.6.0—to be easily installed. Here's the link.
The one caveat is that it didn't update for me the first time—so once I installed the PPA, I went to a terminal window and typed
sudo apt-get update then
sudo apt-get dist-upgrade
and everything was cool.