Do you think NAT is largely responsible for the delay in IPv6 adoption?
In one sense, absolutely. We were facing an IP allocation crisis early on, but now it's been largely resolved. If we didn't have cheap NAT, we would have had to move to IPv6 years ago just to keep up with all the internet connected devices that are proliferating.
However, keep in mind that the infrastructure is what's really preventing change. If infrastructure weren't a problem, then we would have transitioned long ago, even if NAT was available.
NAT was really a solution to avoid upgrading the infrastructure, but it's the infrastructure that is holding us back.
Will we hide behind a single IP address for security?
NAT has given us a certain amount of security, but at a big cost of liberty. I believe we'll see NAT or NAT like devices available for IPv6, but my expectation is that we'll forgo that for more liberty in how we use the internet. Push content is something that has suffered at the hands of NAT, and the iPhone, for instance, is now using a model where internet servers alert it for software and data use.
Firewalls will have characteristics of NATs, and we'll see NAT because that's how ISPs operate, but it will go away as people desire more powerful use of the internet.
When will IPv6 finally get widespread adoption?
It's going to be a gradual process. Europe and Japan already have made significant progress, but there's too much old equipment in the US to switch over quickly.
All the ISP level and above routers and equipment being purchased handles IPv6, but it's going to be about 3-5 years before the old equipment is cleared away enough that ISPs and hosting companies will fell comfortable starting to depreciate IPv4 in favor of IPv6.
The smaller high tech countries will switch first (smaller infrastructure), and the US will lag to a small degree. But I expect in 10 years or so the majority of internet traffic will be routed via IPv6, and most ISPs will have more IPv6 customers than IPv4 only.