Apache 2.0.54-r31 has just gone stable. Perhaps it's time for one of the mailman ebuilds that use the new apache layout to go stable along with it?
Yes, mailman-2.1.5-r4 installs 50_mailman.conf to the old location /etc/apache2/conf/modules.d instead of /etc/apache2/modules.d
mailman-2.1.5-r5 works here on amd64. I guess it's really time to mark it stable.
*** Bug 129006 has been marked as a duplicate of this bug. ***
2.1.8_rc1 is stable, so this should be done