Warning: geek chow.
Google, Yahoo, Facebook and others will test Internet Protocol version 6. Starting at 8pm Eastern Tuesday, these companies will test out how the transition from IPv4 addresses to IPv6 addresses will work. The test will last 24 hours, and all the smart people will be looking for where there might be problems.
From MIT's Technology Review:
One potential technical challenge is "IPv6 brokenness": problems in IPv6 connectivity that prevent users from accessing websites using the newer protocol. When a website such as Google runs two versions of its site for IPv4 and IPv6, it can't properly monitor for brokenness, because users must specifically type in the IPv6 URL to access it. On World IPv6 Day, traffic will be automatically directed to the IPv6 version, which will highlight brokenness and help organizations fix problems."
You probably won't be able to tell anything is going on, but you can go here to get yourself ready for the World IPv6 day.