See the latest news on the kubuntu.org web site as the most up-to-date information shows how to correct this problem:

http://www.kubuntu.org/news/kde-4.2

The problem is that the public key must be installed in GPG.

In this specific case, the key would be downloaded into GPG and added to the APT key ring using this command:

gpg --keyserver keyserver.ubuntu.com --recv-keys 493B3065 && gpg --export -a 493B3065 | sudo apt-key add -

In future cases the required key will be different but the idea will be the same: download the key (gpg --recv-keys,) export it (gpg --export,) and import into apt-get (apt-key add.)