KDE 4.1.1 has been released and packages are available for Kubuntu 8.04, the Hardy Heron. These packages install to /usr/lib/kde4 and can be installed along side your existing KDE 3 installation.

The updated packages for Kubuntu 8.04 are located in the Kubuntu Member's KDE 4 Personal Package Archive (PPA) repositories. To update to KDE 4.1.1, please follow these instructions:
kdeplasma-addons contains new Plasma fun.
Thanks to Harald, Jon, Steve, Guillaume for preparing these packages.
Special thanks to Scott for doing a late night shift to get emergency uploads into Kubuntu Intrepid Alpha 5.
If you want to see your name above, then join the Kubuntu Team.
More information is available in the Kubuntu Wiki