Today when upgrading my Debian Lenny/KDE to the latest version, I started having problems with KDE.
On my first upgrade, I did a simple 'apt-get update', 'apt-get upgrade' sequence. A bunch of packages were held back. The end result was that I could log in to KDE, and could see a desktop, but I had no menu interface.
Considering that there were a bunch of packages being help back, I did a 'apt-get update', 'apt-get dist-upgrade' sequence. Upon logging into the KDE shell, all I saw was a black screen and a shiny obsidian cursor.
It looks like the transition from KDE 3.5 to KDE 4.0 is not seamless in this Debian (Lenny) point release. However, that isn't quite correct. In my /etc/apt/sources.list file I do have entries for testing and experimental. So..., I may now be downloading testing or experimental releases.
In any case, the resolution to the problem appears to be to drop into the console and run one of these three commands: 'apt-get install kde-standard', 'apt-get install kde-minimal', or 'apt-get install kde-full'.