Here are a bunch more, including measurements from CDNs, ISPs, network exchanges, RIRs, the US government, and other large content providers, as well as analysis of publicly available information like AS tables, DNS records, Alexa lists: http://www.worldipv6launch.org/measurements/ http://bgp.he.net/ipv6-progress-report.cgi
My own personal measurements support these claims. I worked on a small academic conference website not too long ago and the percentage of signups from IPv6 addresses approximately matched the traffic % that Google published for that time period. My own website statistics see a higher level of IPv6 traffic than Google shows, though it caters to a more technical crowd so I am not surprised.
10% global IPv6 deployment is not a Google conspiracy. =)