No, I can ping ipv6.google.com from my DIR-868L, and it can be pinged from the WAN side, so it's definitely not an HE issue, nor is it an issue with the router in-between the DIR-868L and my ISP. The DIR-868L just won't forward any IPv6 traffic to the LAN. IPv4 traffic works fine.
(And, I hate to say it, the wireless range in our home was significantly lowered when the DIR-825 was replaced with the newer DIR-868L. And the DIR-825 could at least be hacked with OpenWRT, enabling it to do virtually anything, as it does now, including IPv6 via SixXS. I'm beginning to regret having purchased the DIR-868L.)