At last check the firmware that they pushed out not that long ago, broke HE.net tunnels and would be what I call feature incomplete for IPv6.
To solve the issue for me (and solve another problem I had at the same time, not related to IPv6) I picked up a VPN account and routed my HE tunnel through that. Not the prefect solution but gets the job done.
See following threads for some "light" reading
»
AT&T now blocking IPv6 tunnels»
IPv6 rollouts halted»
IPv6 tunnel still broken on ATT Uverse»
AT&T blocking IPv6 tunnels -- update»
Att U-verse IPv6