You want to be able to run down issues like this if you're going to run
unstable, IMHO. I did an "apt-cache showpkg libcupsys2" to find out
1) that it is indeed present in stable and testing, but not in sid; 2) that the KDE package that depends on it is kdelibs4, which just about
every KDE 3.x program depends on. So then I checked the package page
for kdelibs4 in unstable (through packages.debian.org), and saw that
indeed, it's listed as depending on libcupsys2, which has no installation
candidate. That sounds like a major bug, so I clicked on "Check bug
reports," and sure enough, it's been filed as a "serious policy violation"
bug against kdelibs4 -- seven times (by people who didn't bother to check
the BTS).
A quick google search also turned up this:
http://lists.debian.org/debian-qt-kde/2004/05/msg00294.html