With thanks to the people on this list, I just got 6rd going over Uverse down here in San Diego. I had noticed 6to4 wasn't working, went to investigate and discovered this discussion. Wonders never cease.
My traceroute path to the relay address 188.8.131.52 is as follows:
Delays were a remarkably flat 20-25 ms all the way up; the 20 ms is for the first IP entity on the other side of my VDSL2 link. I have never learned exactly where that is; is it in the cabinet down the street or somewhere in AT&T's vast cloud? THe 25 ms is for hop 8, so there's very little additional latency over what I normally experience from Uverse.
BTW, tracerouting from a static IPv4 address over Uverse has *never* worked; apparently the brain-dead 2WIRE box blocks the returning ICMP error reports. But it does work when you use the NAT in the 2WIRE; go figure.