It Works For Me™
Once you've deployed IPv6, you get to the next exciting part of the process: unfixed bugs in various implementations. For example, OS X 10.6 has an intermittent bug where it will fail to resolve CNAME records that point to AAAA records. A CNAME record is a specific type of DNS record that says "this host name is an alias for another." They are quite often used for virtual hosts, so you only need to update the IP address in one place if you move them all to another machine.
If the host pointed to by the CNAME record is IPv6-only, then OS X 10.6 will sometimes claim that it has no associated address. If, however, you've done a lookup for the target host first (rather than while resolving the CNAME record), then the AAAA record will have been cached, so it will work. This makes the bug somewhat unpredictable in the real world.
Because IPv6 is less common than IPv4, lots of code will not have been properly tested with it, and may fail in subtle ways. This problem is exacerbated by the fact that most current IPv6 hosts are dual stack. If you have a problem with IPv6, most of the time your computer will fall back to IPv4, and you'll just notice a slight delay. Until you try to connect to a remote host that is IPv6-only, then things will break completely, even though they work with everyone else. This makes people quite hesitant to deploy IPv6-only nodes.