This is how looked my desktop some minutes after the kde 4.2 installation.
You need Debian unstable (Sid) and enable the experimental branches to install kde 4.2 version.Have in mind that this Debian branches come with lots of bugs and sometimes your system doesn’t respond as you like. Well, going to the whole point.
Add this to your /etc/apt/sources.list
Enable the experimental branch by adding this to your /etc/apt/sources.list
Update your system:
sudo apt-get update
Install the KDE packages from experimental with the command:
sudo aptitude -t experimental install kde4-minimal
kdebase-runtime: Essential runtime components
kdebase-workspace: Desktop environment
kdebase: Core applications
If you want a complete installation, run:
sudo aptitude -t experimental install kde4
This command will install all packages of KDE 4.2 and could take some time. But, I recommend you install just the packages you need like: kdepim, kdemultimedia, kdenetwork, kdeplasma-addon, etc. Look at the Debian/KDE website for more information.
¡Enjoy your KDE Desktop!
Here are some screnshots of my desktop when playing with KDE 4.2.