I have a 5th gen that I've been running with 7.6.1 and a very similar setup. The clients use the fe80 router address. It all works great.
I went to upgrate to 7.6.4 and I expereinced something similar to what you are seeing. I can ping6 between machines on the local network, but not out to the internet. Although I was assuming that the client machines were operating on cached IPv6 information and that possibly a new machine would not work. I didn't get time to test it as I reverted back to 7.6.1 and without changing anything else it worked once again.
It may not be you.
Thanks for your reply on this. It appears that the 7.6.1 code is not compatible with 6th Generation (http://support.apple.com/kb/HT1998). Assuming that 7.7.1 broke IPv6 routing then this seems like a pretty serious bug. If I had to guess I would say the AE is ignoring the configuration setting for "IPv6 Default Route"
Does Apple have a site for reporting bugs?
Thanks for that link. I suppose is doesnt hurt to ask, we'll see what happens. In the mean time here's a work-around that I was using on a 5th generation AE that also works with the 6th generation AE. It's not pretty, but it works:
Put the AE in Tunnel Mode and build a 6-in-4 tunnel from the Cisco to the AE.
2001:aaaa:bbbb:cccc = /64 on LAN side of router
2001:aaaa:bbbb:dddd = /64 to be used by AE wirless clients
22.214.171.124 = IPv4 address for router LAN interface
126.96.36.199 = IPv4 address for AE
no ip address
ipv6 address 2001:aaaa:bbbb:dddd::2/64
tunnel source 188.8.131.52
tunnel mode ipv6ip
tunnel destination 184.108.40.206
Connect Using: Static
IPv4 Address: 220.127.116.11
Router Address: 18.104.22.168
Configure IPv6: Manually
IPv6 Mode: Tunnel
Enable IPv6 Connection Sharing
IPv6 WAN Address: 2001:aaaa:bbbb:cccc::3
IPv6 Default Route: 2001:aaaa:bbbb:cccc::1
Remote IPv4 Address: 22.214.171.124
IPv6 Delegated Prefix: 2001:aaaa:bbbb:dddd::/64
IPv6 LAN Address: 2001:aaaa:bbbb:dddd::1