According to http://www.gentoo.org/doc/en/security/security-handbook.xml?part=1&chap=6 mount and umount shouldn't be setuid "Switch off the SUID bit on ping, mount, umount, chfn, chsh, newgrp, suidperl, pt_chown and traceroute by executing chmod -s on every file."
If setuid is not set for mount, user mount in fstab like this: /dev/ram0 /mnt/gentoo ext4 user 0 0 fails to mount for a regular user.
I don't want user mount. There should be USE flag to disable it.
then add your own post install hooks via /etc/portage to disable the set*id bits