Kevin Day wrote:

...

9) Once we started publishing AAAA records for a few sites, we started getting complaints from some users that they couldn't reach the sites.

It is possible that a broken 6to4 relay somewhere was causing problems. Running your own local 6to4 relay (rfc3068) will improve performance and reduce the chances of going through a broken one.

Depending upon how many around the world non native tunnels are being utilized. Early on the RIPE folks warned me about tunnels. They were right. -M<