Sounds as if there are a long list of reasons to be wary about Teredo. Here is a document from Microsoft that gives step-by-step instructions for turning it off. The story says: "With Teredo, IPv6 packets are sent as IPv4-based User Datagram Protocol messages to go through IPv4 NATs. Teredo provides IPv6 traffic with address assignment and host-to-host automatic tunneling. A network using Teredo requires Teredo clients, Teredo host-specific relays, Teredo servers and Teredo relays. ... Symantec and Ericsson security experts say they are concerned that Teredo bypasses network security through such devices as firewalls."
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