It works -- kind of.
I'm using the ZyXEL C1000Z modem that they shipped to me and followed the instructions on CenturyLink's website to turn on IPv6 support.
Found two problems so far:
One is with the modem itself.* I can make the modem crash (like, completely stop responding) if I try to visit youtube.com over WiFi with IPv6 enabled. Turn off IPv6 and it won't crash the modem, or plug in instead of using WiFi and it won't crash the modem.
The second problem is that it seems like large packets don't seem to make it through. This seems to have something to do with that the 6rd packets are wrapped up in a packet that also has PPP and PPPoE headers. And so IPv6 is actually having a tough time with its PMTU detection because it needs to actually detect that its available path should be smaller. And sure enough, setting the MTU manually** to 1472 (-20 for 6rd, and -8 for PPPoE) allows YouTube to play videos over IPv6 and lets me download files from IPv6 servers, etc.
I do wonder if ipv6.google.com has manually set their MTU size to the minimum -- because it always seemed to work.
The MTU *workaround* seems to fix the C1000Z modem crash over WiFi as well. In my opinion, a modem should never crash in a situation like that. Discard a packet if needed? Sure. But crash so hard that I have to unplug it? Never.
If someone understands PMTU better than me (wouldn't be hard) I'd love to hear why it doesn't seem to be working and we have to manually set the MTU on the end device for it to work in this case. Is the modem or 6rd gateway just ignoring the ICMP packets that it needs?
* I am running the latest firmware to date: CZC001-4.12.008.11
** On Windows 7, it would be something like this: netsh interface ipv6 set subinterface "Local Area Connection" mtu=1472
You could add store=persistent if you want it to remain after a reboot. You would also have to do this for each "subinterface" you want to use.
Note that this is kind of a hack/workaround, and I'm don't understand PMTU well enough to know why it's not handling correctly (the modem has a sit0 interface that is correctly set to an MTU of 1472). I just know it'd be best in my mind for it to happen in the modem or 6rd gateway since it knows it is doing 6rd and PPPoE, where your computer shouldn't have to care about that sort of thing being upstream.
Another post here: »6rd only semi-working...
And here: »Re: 6rd is long as live on at&t!