How available is IPv6 handset to handset connectivity? I understand that T-Mobile offers it, and Verizon can but it's usually disabled. If you have a true peer to peer connection without NAT, more options become possible.
If you can get a peer-to-peer connection, it's irrelevant whether you're behind a NAT or not. That's what ICE is for: to get a peer-to-peer connection even if you're behind a NAT. Having publicly routable IPv6 addresses may increase the likelihood of getting a peer-to-peer connection, but it's not a prerequisite.
