After emerging slim on arch i686 there was no /etc/init.d/slim file which caused the command: rc-update add xdm default to return with the error. The emerged slim Version ist recorder as v1.3.1 via command: slim -v. This all happend in the x86 Branch. Reproducible: Always
Just change DISPLAYMANAGER to 'slim' in /etc/conf.d/xdm just like the postinst message of slim told you to