Kde 4.6 was released on January 26, 2011. If you want to try this new version you can follow this simple steps.
Add backports repository
sudo add-apt-repository ppa:kubuntu-ppa/backports
Update and upgrade the system
sudo apt-get update && sudo apt-get upgrade
sudo apt-get dist-upgrade
After reboot you can see Kde 4.6 in action.