Google is selective about exposing IPv6 DNS entries. Your ISP has to be in the Google IPv6 program which means that Google trusts your ISP to provide quality IPv6 connectivity. Basically, Google don't want half-assed IPv6 networking setups tainting their brand as unreliable if for instance Bing works (having only IPv4), but Google is slow or inaccessible.

It's understandable from a marketing point of view, but IMO a pretty coward stance on employing new technology. I'm using Google services over IPv6, but the reason I can do that is because I run my own bind instance and have specifically added the Google IPv6 forwarding entries:

Should you have IPv6 and bind, here is what they look like:

    zone "google.com" IN {
        type forward;
        forward first;
        forwarders {
            2001:4de0:1000:a4::2;
            2001:4de0:1000:a3::2;
        };
    };

Other than that, reasons for not reaching IPv6-sites, even if you have IPv6 enabled is that (at least) Chrome is very selective about IPv6 support. It may accept IPv6-only sites if the position of the moon is right relative to the suns orbit around the milky way.

With Firefox this isn't really an issue, but Google has tried really hard to avoid IPv6 being used with Chrome. For instance: If you are on a tunnelled IPv6 connection (i.e. non-native, like teredo) it will attempt to detect this and block IPv6-connectivity because Google has chosen for you that this may provide an inferior web-experience with higher latency compared to plain IPv4, if available.

Google's IPv6-efforts so far, while I could give them credit for at least have taken a few baby-steps, is by far the most cowardly I have seen anyone take. I find it very hard to applaud them.