Just saw this. I'll repost my findings from the other thread here. Many thanks to NetDog for all the help.
I couldn't get IPv6 working with my RT-N66U. I knew IPv6 was enabled in my neighborhood thanks to NetDog, and it was working fine when my computers were hooked directly to the modem. So the next step was to look at the router. I was on version 260 of the firmware, and even tried downgrading to 220 I believe. Neither worked. So I switched a Merlin build of the ASUS firmware (246.20) and everything is working perfectly now.
You can get the Merlin builds here: