Oops, should have linked to the bug report itself.
If I recall RFC1034, if CNAME record is present, no other data should be present (check section 3.6.2).
There's no problem querying the server, something goes wrong somewhere in-between the query and whatever API the Aqua applications are using to retrieve the result.
(it was ignoring AAAA records if A records were returned for the address more than 125ms faster)
It's actually much much worse than that--GUI apps (though not command line apps) will almost always ignore AAAA records if the host has a CNAME record, try shutting off IPv4 in network preferences and see for yourself. (You'll have to set an IPv6 DNS server manually, naturally)
All the main Google and Youtube servers become inaccessible in Safari and Firefox (my DNS provider has Google's AAAA records so that's not the issue), along with roughly 2/3s of the IPv6 web servers and 1/2 of the download mirrors.
Ars Technica's IPv6 columnist has been banging the drum about this for months now, so far there's scarcely been any acknowledgment on Apple's part that the problem even exists. (Admittedly I haven't tested it since the last update, though.)
Even Mars which presumably could be made habitable would have a latency on average of a bit more than 14 minutes each way.
For that reason I think for the Solar System we'll need to forget about IP altogether and go back to UUCP.
I know I am not using IPv6 at the moment. How can I test whether it is my setup that fails, or my ISP that fails (or any other part)?
If you're using Windows 7 or a recent Linux, your PC supports it. Mac OS X Leopard will also (it's broken in Snow Leopard unfortunately).
If you're using a router, it probably doesn't support it although you may get lucky. There are a handful of IPv6 WiFi routers on the market, luckily they are all mainstream and pretty widely available.
A few ISPs offer it--you may check with them, but if you're plugged in directly (not through a router) and are using one of the above operating systems and still aren't seeing it then you can be fairly sure that they don't. If not, you can still get IPv6 through a static tunnel (eg from from HE) if you're located close to a tunnel server. If you do sign up with HE you'll want to point your PC or router to their DNS servers in order to connect to Google and Youtube over IPv6. (Google only offers that service to ISPs, such as HE, that have specifically requested it.)
Also the DIR-825.
And the DIR-615.
And the Airport Extreme and Time Capsule (no PPPoE with these though).
I don't live in the U.K., but I had no trouble finding any of these for sale in my area.
(There's even a Wikipedia page.)