KDE 4.0 stable is now available for Linux. This new release of KDE 4 includes a whole lot of new improvements like a whole new desktop shell Plasma, better look and feel with Oxygen artwork, hardware acceleration in KWin, a new document file viewer Okular, improved KDE applications and lot more.
Here is how you can install KDE 4 on your existing Ubuntu Gutsy Gibbon install. 
- First remove any previous KDE 4 packages you have installed. Fire up a terminal window and type sudo apt-get remove kdelibs5 kde4base-data kde4libs-data
- Then open up your sources list with /etc/apt/sources.list
- At the end of the file add deb http://ppa.launchpad.net/kubuntu-members-kde4/ubuntu gutsy main
- Now run apt-get update
- Next to install KDE4 type in terminal apt-get install kde4-core
- You will get a warning that the packages are not authenticated. Hit Y for that warning.
- Log out and then you will be able to run a full session by selecting “KDE 4″ from your login manager.
Thats it. I am installing KDE 4 now on my Gutsy install.
Will post I have posted some screenshots of KDE 4 when its over. 
Finn
24 Jul 08 at 8:09 pm
Is it really stable? If so, is it available for Linspire 6?
Manas
24 Jul 08 at 8:32 pm
Hmm… It is not as stable as KDE 3.5. You will have to wait for later bug fix releases (KDE 4.1) to be more stable.
About Linspire, not much idea. I don’t use it.