Here is how it install KDE 4.5 in Ubuntu 10.04 Lucid Lynx system.

Step 1. Add the Kubuntu-ppa backports repository and then update the repository listing.

$ sudo apt-add-repository ppa:kubuntu-ppa/backports
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /etc/apt/secring.gpg --trustdb-name /etc/apt/trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyserver keyserver.ubuntu.com --recv E4DFEC907DEDA4B8A670E8042836CB0A8AC93F7A
gpg: requesting key 8AC93F7A from hkp server keyserver.ubuntu.com
gpg: key 8AC93F7A: "Launchpad Kubuntu Updates" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1
$ sudo apt-get update

Step 2. Install kubuntu-desktop package.

$ sudo apt-get install kubuntu-desktop

Durning the installation you will be prompted to either use GDM or KDM as your login manager. It is totally up to you as to which one you pick. You will be able to get to either Gnome or KDE with either.

Sit back and relax and in a short time you will have latest version KDE installed along side your Gnome desktop manager.