Some time ago, I wrote about the problems I had getting IPv6 to work through my AT&T DSL connection.
It turns out that the problem is the crappy DSL modem that AT&T gives to their customers. It is a Siemens Speedstream 4100. Siemens got out of the DSL modem business, and there are no firmware updates for this DSL modem available. On top of that, SBC apparently provided their own configuration, which may have crippled the modem even more.
So, today I finally went out and bought another DSL modem, a D-Link DSL-2320B. With that, I can use IPv6 through a tunnel at HE's tunnelbroker service just fine.
I am actually thinking of returning this DSL modem, though, since it has some other ridiculous setting that is unfortunately not configurable: it renews the IP address every 15 seconds or so... filling up my log files for no good reason. I'll never understand why they can't just provide a configuration option for something simple as that. Even the Siemens DSL modem has that...