Many big content providers are quite reluctant to enable their services over IPv6. The reason for this is that in many cases IPv6 connectivity does not yet have the same production level quality as IPv4 has. If for example amazon, ebay, google, etc would add AAAA records (IPv6 AAAA = IPv4 A record) for their websites, there's a possibility that some of your users will try to access your website over IPv6. In some cases the clients ISP network is not "really" IPv6 ready, IPv6 will only work to some parts of the Internet. The result is that these companies (Google, Amazon, Ebay, etc) will loose custommers and thus revenue. This is a real concern for these companies and for the global IPv6 deployment.
Google already made their search engine reachable over IPv6 using ipv6.google.com and has said to be commited to full IPv6 deployment. They are now taking it one step further. All there services will be accessible over IPv6, once they determined your ISP is IPv6 ready, i.e. meets certain quality requirments! Depending on that the Google DNS server will either return a AAAA (ipv6) or A (IPv4) record. A ingenius system to control wether you will access google over IPv4 or IPv6!
Network not IPv6 ready
Ipv6 ready ISP
Google describes the requirements like this:
To qualify for Google over IPv6, your network must have good IPv6 connectivity to Google. Multiple direct interconnections are preferred, but a direct peering with multiple backup routes through transit or multiple reliable transit connections may be acceptable. Your network must provide and support production-quality IPv6 networking and provide access to a substantial number of IPv6 users.