I just setup someone's U-verse. IPv6 worked. Out of the box. No configuration necessary.
That is news to me... What modem was it?
See I have uverse tv, voice and net and that forces me to a 2wire 3801 modem, and that POS has no IPv6 option and crappy forced firewall.
The last I knew, communication between AT&T and your house was done via IPv6, but the end user had no access to it...I don't know if that's still the case or not