I'm starting to roll out IPv6 across our network and I would like a way to tell from within wpad.dat if a given host/url is IPv6-only (like ipv6.google.com). The reason that I ask is because our proxy server (Astaro Security Gateway v7) is IPv4-only and returns an error page if you try and access an IPv6-only host through it. The next version of our proxy server supports IPv6 but it may be a while before we are able to upgrade.
Most of our users are using Internet Explorer - our standard is IE7 but there are quite a few people still using IE6 and there are also a lot of people using Firefox, Chrome, and Safari, and who knows what else.
wpad.dat gives a javascript function. You will have to look up the IPv6-support for functions like isInNet or their ipv6-capable complements.