I had the same problem, black screen with cursor after login in kdm.
Solution: I had deinstalled dragon player, which deinstalled kubuntu-desktop.
By reinstalling (apt-get install kubuntu-dektop), I saw I had 2 other missings packages to install: kdebase-workspace, plasma-desktop. After that, everything went smooth, without deleting .kde folder ;)
I had the same problem, black screen with cursor after login in kdm.
Solution: I had deinstalled dragon player, which deinstalled kubuntu-desktop.
By reinstalling (apt-get install kubuntu-dektop), I saw I had 2 other missings packages to install: kdebase-workspace, plasma-desktop. After that, everything went smooth, without deleting .kde folder ;)
-MR