http://youtube-global.blogspot.com/2010/02/youtube-calls-on-ipv6.html

<<Since the very first announcement of ipv6.google.com (IPv6 connection required; if you don't have it, ask your ISP to deploy it), we have been committed to supporting IPv6 and have steadily added IPv6 support to more and more services. The service most requested to have IPv6 support has unquestionably been YouTube. Given all of this, we're proud to make YouTube available over IPv6 and to begin streaming videos from a select number of sites worldwide to our Google over IPv6 partners. With YouTube on board, we now have a significant amount of content delivered on IPv6 and a real audience/traffic for it. This is a good day for YouTube, our users and for an open and accessible Internet.>>