KDE 4.3 was released just yesterday, and it comes with a lot of great new features and improvements, like the the new Plasma theme called Air, new Plasma widgets, several improvements to Dolphin, the default KDE4 file manager, improvements to the KDE games, and these are just a tiny part of all the improvements KDE4 introduced. The full announcement including reader's changelog can be found on the official KDE4 website, here.
Kubuntu comes with an older release of KDE4, but there is a PPA (Personal Package Archives) available which includes the newest packages for Jaunty, so in order to install it just follow the steps below.
First, open up a terminal like Konsole or GNOME Terminal and edit the sources.list file using your favourite text editor, e.g.:
sudo nano /etc/apt/sources.list
Or, for Kate:
sudo kdesu /etc/apt/sources.list
If you are installing KDE from Ubuntu, you can use Gedit:
sudo gedit /etc/apt/sources.list
Enter your user password and add the following repository line:
deb http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu jaunty main
Next, make sure to save the file (Ctrl+O followed by Ctrl+X in Nano) and update the package lists:
sudo apt-get update
All you have to do now is to install the new KDE 4.3 environment by issuing:
sudo apt-get dist-upgrade
Should take a while. This will fetch and install all the newer packages from the freshly added repository. To see the new KDE 4.3 installation log out and log back in KDE, or open up Konsole and type:
sudo /etc/init.d/kdm restart