in addition to the regular IPv4 flavor RTMFP connectivity checker "cc.rtmfp.net", i have set up an experimental version of the server at my house for IPv6.
if you go to http://www.rtmfp.net/ and select "IPv6 connectivity checker", it will attempt to run the RTMFP connectivity check for IPv6. please give it a try if you're on the IPv6 Internet (or if you're not sure whether you are). we are curious to see the characteristics of real-world IPv6 deployments.
note that the checker runs at my house and is not a production service. also, my IPv6 connectivity is via 6to4, so even if you have regular IPv6 connectivity, you may not be able to reach the 6to4 section of the network (2002::/16) and therefore the check may time out.