This isn't my field, so I could be about to make a fool of myself, but... Watching some NANOG lists I've gotten the distinct impression that the biggest hurdle

This isn't my field, so I could be about to make a fool of myself, but... Watching some NANOG lists I've gotten the distinct impression that the biggest hurdle at this point to large scale deployment of IPv6 at the carrier/etc. level is budgets. The tech is (mostly?) all here to support it, working, from multiple vendors, but it's simply different than whats deployed in many cases. When many of these big networks should have been deploying more transitional technology over time they were busy freaking out and spending 0 money in the recession. Now they're looking at 4 years worth of capex + 100% new customer premise and so they drag their feet some more. Vendors, of course, see this as a huge opportunity so they're maybe not as keen on backporting code to older equipment as they should be. So carrier NAT and overlapping numbering suddenly looks real good to everyone. Except for the network engineers. End user equipment (esp things like phones, blu-ray players) is a different story in terms of readiness, but this can be handled at the border.