hsw yep, you need ISC dhclient too in order to get DHCPv6, see net/dual-dhclient
Once installed, something like this in /etc/rc.conf should do the trick:
ipv6_activate_all_interfaces="YES"
ip6addrctl_policy="ipv6_prefer"
ifconfig_$if="DHCP"
ifconfig_$if_ipv6="inet6 accept_rtadv"
dhclient_program="/usr/local/sbin/dual-dhclient"
rtsold_enable="YES"
rtsold_flags="-aF"
Were $if is your network interface
Hope this helps ; )