It is very hard to fix the system that is built from mixing distribution(testing/unstable). There is limitation in the apt-get dependency logic which from my experiencel only support one way upgrade, despite how flexible people are claiming about apt-get.

you said you have apt-get setup correctly because you can pull in some other packages, the truth is, the only way apt-get can be 100% sure about setup correctly is 'stable' without any third party backport. The reason that you can have some packages work is that they have less dependency on others. KDE is almost an acid test of how 'broken' a distribution is, well I have seen other situation that is even worse(like I have installed a woody system without locale package, add Xfree86 4.3 backport which has a newer debconf then later when I want to add back locale, apt-get barks that the debconf is too new!).

The only suggestion I can think of is to do a apt-get dist-upgrade having 'unstable' as the only source(I assume you have a KNOPPIX hd-install) then retry what you wanted to do. Make sure you remove every other source site other than ftp.debian.org.

I have forgotten how many times I have deboostrap from scratch(I use woody + backports) in ironing out these weird dependency problems in apt-get.