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

Then

sudo apt-get dist-upgrade

After reboot you can see Kde 4.6 in action.