Perhaps you can resinstall the gnome package that's supposed to set those things up. No really useful advice though.

As a late entry to this, I think what you experienced was quite possibly due to the /etc/xdg changes that happened recently in -current. The sysconfdir for kde and xfce is now /etc/kde and /etc/xfce instead of /etc/xdg, and that's not expected to change again, so yes, reinstalling kde and/or xfce packages should take care of it without further issues. One of the hazards of running -current is this sort of thing -- it's a testing ground, and sometimes things are done one way, but a better way is later found and thus they change... :-)