IPv4 and IPv6 users alike

[cd34]

Teredo is Windows 7 built in tunnel in conjunction with a number of providers. By default, it is enabled, but, DNS resolution is turned off so that you'll have ipv6 for any site that is specifically ipv6.

http://ipv6-test.com/ does something similar and will tell you what your status is. If it says IPv6 is configured, but IPv4 is preferred, then you probably have a Teredo tunnel enabled on your machine. I believe it is the default for Windows 7. There is a registry entry (which I wouldn't suggest unless you were really curious) that you can flip that means you can hit dualstack sites and will prefer ipv6.

http://ipv6.google.com/ is one other destination that is a reasonably good test.