Are there any security concerns having IPv6 and Teredo tunneling enabled in Win Vista/7? AFAIK, using Teredo basically allows bypassing of the firewall, as the firewall cannot really filter the contained IPv6 packets. (By firewall, I refer to the network firewall, not the personal Windows firewall.)
If upgrading the firewall is not an alternative, what can be done to keep the network as secure as possible?