OK, I see that now - someone upstream must be advertising 188.8.131.52.
Is this normal for Windows 7 or Macs now to try 6to4 without human intervention ? They are probably going to get worse connectivity to dual-homed servers than going through IPv4, apart from the firewall-bypassing issues.
Looks like this laptop is in fact sending ip6 router advertisements around the LAN. Have to chase them down and find out why.
I'm pretty sure 6to4 is on by default on Macs and I know Teredo and I think 6to4 is on in Windows 7.
184.108.40.206 is an anycast address for 6to4 relays, so anyone running a relay is advertising that, as well as the 2002::/16 space.