In the aftermath of the World IPv6 Day YouTube seems to be serving its content over IPv6 now. Interestingly the frontpage is still served via IPv4 (if you're not in a Google IPv6 whitelisted network). But all the Flash and HTML5 video content is served through IPv6 if available, as the cache servers return proper AAAA DNS records. Apparently that's the case unless your network is blacklisted because of bad IPv6 support and even if Google has some caches at your provider's site (which is the case for Alice DSL in Germany, at least).
I think that's quite some motivation for the providers to at least fix IPv6 connectivity if available and to suppress rogue IPv6 router advertisements in their networks. I had to ensure the former today and the latter is a constant source of grief with the bulk of L2 switches and Wi-Fi access points not being IPv6 ready.