When I rebooted my firewall the other day I ended up not being able to route via IPv6. This seemed very odd since route -n --inet6 seemed very clear that I had a default route.

I eventually found that this is a kernel bug through a message on Sixxs' forums.

So the solution for me was:
ip -6 route add 2000::/3 via 2001:4830:1600:bc::1 dev sixxs

and then add a static route at boot time:
# cat /etc/sysconfig/static-routes-ipv6
sixxs 2000::/3 2001:4830:1600:bc::1