OpenVPN with IPv6 is now supported.
Configure the IPv6 networks on the OpenVPN Server settings page. Create IPv6 firewall rules on the Firewall Rules OpenVPN server interface tab.
Install the openvpn installer with IPv6 support.
http://www.greenie.net/ipv6/openvpn.html
http://www.greenie.net/ipv6/openvpn-2.2.0-ipv6-savepasswd-20110522-install.exe
That should be enough. Client support is limited to windows with the modified installer and any other that has the IPv6 payload patches integrated.
I've contacted the Viscosity folks but it's not in the pipeline yet. They are aware of the patches though.