This page will walk you through getting IPv6 to work correctly on your Cisco device

Enable IPv6 routing on your router

#conf t
#ipv6 source-route
#ipv6 unicast-routing
#ipv6 cef
#ipv6 multicast-routing
#ipv6 route ::/0 Dialer0

Enable IPv6 to work on your internal Ethernet Ports

#conf t
#interface FastEthernet 0/0
#ipv6 address <your_slash_48>:1::/64 eui-64
#ipv6 enable
#ipv6 nd prefix <your_slash_48>:1::/64
#ipv6 nd managed-config-flag
#ipv6 nd router-preference High
#ipv6 nd ra interval 60

Enable IPv6 to work on your WAN side

#conf t
#interface dialer0
#ipv6 enable
#ipv6 traffic-filter adsl-ipv6 in

The above config didn't work for me I had to create a new /64 via Clueless and add this here as an IP address

ipv6 nd prefix <your_slash_64>::1/64

I would also add the following traffic-filter to the dialer interface

ipv6 traffic-filter outboundfilters-ipv6 out

Lock down your IPv6 network with an access list

#conf t
#ipv6 access-list ipv6 adsl-ipv6
#permit tcp any any established
#permit icmp any any
#deny ipv6 any any
#interface dialer0
#ipv6 traffic-filter adsl-ipv6 in