Twitter is up. I still can't get NYT - down for over a day? wow

How difficult of a thing is this to pull off?

If you can compromise a shared registrar, pretty trivial.

Well, the "attacking multiple sites at once" is trivial. The "compromising a major registry" is at least supposed to be kinda hard.

You'd have to have the ability to change DNS records for their domains. If you can point "whatever.com" to a NS that you control, it's game over until they take it back.