An error I received after trying to update one of my Kubuntu Karmic Koala installations to the latest KDE 4.4 was plasma desktop has no installation candidate.
The solution was to recheck all of the repositories listed in /etc/apt/sources.list and the files in /etc/apt/soruces.list. I previously had a kubuntu beta repository. I exchanged this for the correct stable repository, updated, ran an upgrade. Additionally, I needed to run sudo apt-get install kubuntu-desktop for the appropriate packages containing "plasma-desktop" were installed.
You can actually start plasma-desktop from Kwin by pressing Alt+F2 and typing "plasma-desktop", or it can be automatically be restarted by switching to one of the virtual consoles and typing restart kdm, but beware this will kill any gui applications you have running.