[22sept11]: UPDATE: It appears that I may be wrong as I've now seen my machines automatically obtain an IPv6 address now (using an RS and receiving an RA). Possibly what was needed was a full power cycle of the Time Capsule to effect the IPv6 configuration.
After carefully managing to set up IPv6 on my Time Capsule using a manually configured tunnel I noticed that it had stopped working after updating to 7.5.2. I hadn't had the time to look into it before, but now I have it seems that 7.5.X has some kind of 'safety' feature that disables Router Advertisements (RAs) on your LAN when another box on your LAN is doing the DHCP (or RA) which seems DUMB. It should be configurable as alot of people don't use their TC as their main router box - since it only supports PPPoE. If someone's going to bother manually setting up a tunnel then they probably want to use it...
It seems that the Time Capsule reports this an "IPv6 Tunnel Error" - despite the fact that actual tunnel is up - so this error must be indicating it has had to stop the RA service.
I now have to manually add the IPv6 route to my hosts which make IPv6 a bit of joke on things like my iPhone or other devices I can't configure manually for IPv6.
Nice one Apple - just when we thought IPv6 was getting easier!