[zaphoyd]

The risk is much smaller today than it was when the ipv6 whitelists for google, etc were originally set up. All patched modern operating systems and browsers have corrected the behavior that originally caused this (preferring automatic v6 tunnels over native v4) so that the only brokenness remaining in theory is outdated software and improperly configured manual tunnels.

See http://fud.no/ipv6/ for more information on measurements of this issue as browsers and oses fixed this problem.