My tunnel was working perfectly under 10.6 (10/10 on test-ipv6.com.) After installing 10.7, with the same setup I get 0/10 and this repeated message in the kernel log:

nd6_lookup: failed to lookup (the IPv6 address of my gif0 interface)

In spite of that, I'm able to ping6 the address of the remote endpoint. Thanks for any suggestions.