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In Mac OS X 10.2 Jaguar, Apple introduced IPv6 support. Like other IPv6-capable operating systems, Mac OS will prefer IPv6 over IPv4 if it has a choice. That is, until yesterday's 10.6.5 Snow Leopard update. With regular IPv6 connectivity, the newly updated Snow Leopard will still try to connect over IPv6 first, but for IPv6 destinations that are reachable over 6to4, the snowy cat prefers IPv4 instead. It will only connect over 6to4 to IPv6 destinations if there's no IPv4.

The apparent rationale for this move is that 6to4 is responsible for a disproportionate share of non-working IPv6 setups. Measurements by Google show that simply giving www.google.com an IPv6 address means that, subsequently, about 0.1 percent of all Google's users would be unable to connect to the search giant. That is unacceptable to them, not just from a commercial perspective, but also... how are you going to debug this problem if you're unable to search the Web? ISPs are unlikely to hold user's hands if this eventuality comes to pass. Earlier results show that Mac 6to4 users are a huge proportion of the (currently) 0.3 percent of IPv6-capable users. But 6to4 has exactly the properties that make it fail in ways that tend to go unfixed.

ARS Technica has the article HERE!