freitasm: Nope, cancel that. I am now inclined to believe it's Opera Mobile proxy. Turn off Opera Turbo and it's always IPv4. Using stock browser I always get an IPv4 address.
MattR: Yep, I only see IPv6 on my Galaxy Nexus if I'm using my wifi (Snap native IPv6). Turn off wifi and I'm back to v4 only.
Even on wifi, http://www.apnic.net shows my v4 address, but http://ipv6.internode.on.net shows v6