udev looks for rules in 2 locations /etc/udev/rules.d/ and /lib/udev/rules.d/. system defined rules are supposed to be installed in /lib/udev/rules.d/ and not in etc, however modemmanager installs the rules into /etc/udev/rules.d/. this should be fixable by replacing the line (in the ebuild): --with-udev-base-dir=/etc/udev/ with: --with-udev-base-dir=/$(get_libdir)/udev/
modemmanager-0.5.2.0 installs them at proper location