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.

You should be able to setup a GIF interface on the LAN I'd guess and configure another box on the LAN with a tunnel as well. No idea why you would want to do that though.

No. I mean clientes outside the LAN. Internet (ipv4'd) clientes.

@Koen:

Based on the topic title I believe what is meant here would be setting up an OpenVPN/PPTP/L2TP tunnel between two IPv4 endpoints and route IPv6 traffic over it. I could use this to allow IPv6 traffic from an IPv4 internet connection by tunneling it via my home network. Should that be possible with the current functionality available?

That would be the ideal situation, as I'm using OpenVPN too: remote clients with openvpn tunneling ipv6 traffic via their vpn connection. Or, directly, establishing the tunnel via any of the IPv4 ip's of the WAN interface. Why? Simple: the ipv6 connectivity is native, so people don't have to setup tunnels against far ipv4 endpoints, but against this near one :)

Just assign a ipv6 address on the downstream router and create a static route on upstream for the networks on that router.