If you install kdelibs rc7, the two following files: lib/kde3/plugins/designer/kdewidgets.so lib/kde3/plugins/designer/kdewidgets.la Gets installed in the KDEDIR. So if you install kdelibs 3.1 the plugins gets install in /usr/kde/3 if you were running that KDE version before or /usr/kde/cvs if you were running the KDE cvs version. I believe that the plugins should follow the same path as the KDELIBS version being installed. As a work-around, using KDELIBSDIR allowed me to get those properly installed into the wanted path. Discussion with Dan Armak indicates that the KDELIBSDIR shouldn't have effect on the kdelibs module.
Didn't you eventually say on the m/l that this was resolved and was due to somne configuration mistake of yours? For clarification, KDEDIR from the environment cannot affect the emerging of kdelibs or any other kde package. The eclasses set KDEDIR to the right value; they always set it to something and they don't care about its environment value. THat's why we have special variables like KDELIBSDIR about which they do care. So kde3.0.x being installed cannot (in theory) affect the emerging of kde 3.1 at all.
This has been resolved. New kdelibs now install the qt plugins inside the prefix and no more inside the active KDEDIR.