"Google has been supporting IPv6 since early 2008, when we first began offering search over IPv6. Since then we've brought IPv6 support to YouTube and have been helping ISPs enable Google over IPv6 by default for their users. On World IPv6 Day, we'll be taking the next big step. Together with major web companies such as Facebook and Yahoo!, we will enable IPv6 on our main websites for 24 hours. This is a crucial phase in the transition, because while IPv6 is widely deployed in many networks, it's never been used at such a large scale before."
Google expects that less than 0.5% of the users will be affected by the test. There's even a page that tests your connection and detects IPv6 connectivity issues. Even if IPv6 will be enabled for 24 hours, IPv4 will continue to be available.
{ via Blogoscoped Forum }