Can anyone elaborate on what he means when he says that Verisign can 'go down' and take down most of the internet with it? How would a registrar going down affect anything to do with actual hosts?
If coordinated with an attack against the root nameservers so we couldn't change the .com and .net nameservers, DNS would become a real disaster. If combined with some BGP trickery, you could even see domain names being poisoned.
We should be able to be worked around the damage eventually; but so much of the internet relies on so few root servers/hosts/routers.
When you look up google.com, these root nameservers are queried for com, and they return the results (name and IP) for the nameservers for .com
These nameservers for com are then queried for google.com, which then return the results for the nameservers for google.com.
Google's nameservers are then queried for google.com, and an IP is returned.
So yes, given how DNS works, all .com and .net domains would stop resolving if the Verisign nameservers for .com and .net were to go down. Most people go through caching nameservers, which would retain the values for google.com, and continue to return them, up until the time to live on those records expired, at which point they too would stop returning any values if the upstream servers hadn't returned before then.