At least two of these root name servers are not pingable.
a - Does this really matter? (the sky has not fallen on us)
b - Why are they not pingable and how often does this happen?
c - How many root name servers would it take for the net to become "unusable"?

Nope; just because they're not pingable doesn't mean they aren't working.

Their firewall can prevent ping's from getting through. Network Admins do this to cut down the resource usage of responding to a ping (it's not much, but if you get millions per second, it all adds up. Some DOS attacks use ping floods). It depends on the net admin.

Good question that I don't know off hand. They have been attacked before rendering some of them unreachable, but I don't think all 13 have been brought down at the same time. You may google about it on the net, there's probably some cool articles on it.

Thanks for the answer. Initially I had `traceroute -A` instead of `ping -c2` in that one-liner. from the output of the traceroute I could guess that some boxes were firewalled and also not repsonding to IMCP(?) pings.

Replace "ping -c3" with: