If you mean the "ipv6 day" thing, the only problem that will occur is that people who have IPv6 stacks enabled but broken connectivity will suddenly have long timeouts for host which add AAAA records which will result in ISPs getting a large amount of phonecalls for those, for the rest nothing will change.

I don't usually stick up for MS, but the only brokenness I know of with MS is: 1) Internet Connection Sharing and rogue router advertisements (very bad for the LAN!) 2) Opera did some bad things for a while on Windows. One of the larger sources of broken clients is Macs running 6to4 and not running the most recent version(s) of Snow Leopard. Many of these are behind security devices that block 6to4, or they just have crummy 6to4 service where they are on the network. Either way, they preferred 6to4 connectivity over IPv4 in all but the most recent versions of OS X (note that there is a non-trivial install base that can't be upgraded as 10.6.xx doesn't support all Macs), even when the 6to4 was broken. In other words, Mac OS used to not properly implement RFC 3484. http://ripe61.ripe.net/presentations/162-ripe61.pdf Certainly there are plenty of Windows machines with broken IPv6 connectivity (for someone like Google, that might be tens of thousands of users), but there are far, far more broken Macs in absolute numbers. And there used to be even more users running Opera on Windows, which was broken. For more details of what broken implementations do: http://www.getipv6.info/index.php/Customer_problems_that_could_occur At the end of the day, though, it doesn't matter if Windows or Mac or Linux is better. What matters is that we have to get people to fix these broken implementations before anyone will want to publish AAAA records for public web sites.