IPvFoo is a Chrome extension that adds an icon to your location bar, indicating whether the current page was fetched using IPv4 or IPv6. When you click the icon, a pop-up appears, listing the IP address for each domain that served the page elements.
Everything is captured privately using the webRequest API (new in Chrome 17), without creating any additional network traffic.
https://chrome.google.com/webstore/detail/ecanpcehffngcegjmadlcijfolapggal
IPvFoo now runs on Firefox, but there are a few bugs to work out.