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Packages need testing before they are ready to go into the official repositories and sometimes it is not so easy to create e.g. official backports of the newest KDE version, especially, when it is a beta version. Launchpad offers Personal Package Archives, which are great for testing and it is easier to upload packages there. You should bring to your attention, that the packages in there are mostly not tested in a long release cycle like the official ones and there might be not the same support as well.

There are three Kubuntu PPAs:

Updates:

Updates for Kubuntu releases which are due to go to Ubuntu Updates. Mostly KDE point releases.

deb http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu jaunty main 
#deb-src http://ppa.launchpad.net/kubuntu-ppa/ppa/ubuntu jaunty main

Backports:

Backports of new versions of KDE for Kubuntu which are not yet tested enough to go to Ubuntu Backports.

deb http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu karmic main
#deb-src http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu karmic main

Experimental:

Testing ground for Kubuntu packages.

deb http://ppa.launchpad.net/kubuntu-ppa/experimental/ubuntu jaunty main
#deb-src http://ppa.launchpad.net/kubuntu-ppa/experimental/ubuntu jaunty main

You have to add the needed PPA to your /etc/apt/sources.list either using kpackagekit or an editor. After that do a sudo apt-get update in an open terminal.

You will notice a warning about a missing key:

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY <KEYID>

Packages are signed with a key for security reasons and it is poor security practice to ignore the warning. So you have to import the key with:

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com <KEYID>

After that do another sudo apt-get update and the the warning should be gone.