lammert - 6:59 pm on Mar 27, 2010 (gmt 0)
For clarification, IPv4 is our current IP address numbering scheme with IP addresses of the form xx.yy.zz.aa. There are not much left of them and the numbering scheme of IPv6 has been proposed to create a huge amount of new IP addresses. The 0.078% of Internet users Yahoo talks about are those internet users with systems which claim to understand IPv6, but in fact don't and in reality are only able to speak to other computers over the IPv4 level.
The hack Yahoo proposes is to let the caching DNS server of the ISP between that broken system and the internet translate all DNS queries for IPv6 addresses to IPv4 address queries. This is not only a technical issue, but also an issue of trust and security. If an ISP gets the rights and technical facilities to modify DNS queries and responses to and from upstream authorative DNS servers on the fly, there is also the fear of modification in other situations. Untouched translation of domain names to IP addresses and vice versa is one of the important trust issues of the Internet.