From my .xsession-errors: startkde: Starting up... /usr/kde/3.5/bin/startkde: line 371: xprop: command not found kbuildsycoca running... And yes. xprop isn't installed as nothing (at least not startkde) depends on it. Doesn't seem to hurt much though.
Thanks, dependency added to kdebase-startkde and kdebase monolithic.