Basically, Hurricane Electric and Cogent (two major transit providers) are refusing to exchange IPv6 traffic. That breaks stuff. For me the problem manifested as a user facing a 20 second delay whenever he ssh'd in (over IPv4). (I'm on Hurricane Electric. The reverse DNS for his IPv4 address is served by Cogent DNS servers. Reverse DNS lookups were trying to access the Cogent name server over IPv6 and had to time out before IPv4 was tried.)
Other than that though, I actually haven't encountered very much IPv6 breakage.
No, mainly because it would be business suicide for whoever is involved. Especially if neither party is willing to pay for transit to the other... IPv6 is still small enough that it doesn't yet force one providers hand over the other.
That being said, if you are single homed on HE only then I would suggest you purchase transit elsewhere as well, or find a datacenter that has multiple transit providers. Being single home is dangerous.
The real question is going to be how long can Cogent hold-out. HE is willing to provide them free IPv6 peering, why not accept it so that their customers can get a full IPv6 table...
