I don't think anyone has ever succeeded in getting Chord to work on a large scale. It is a relatively "brittle" system in that it requires a very specific topology which constantly must be updated as nodes enter and leave the system. So this is probably not how a successful massive DDOS effort would be architected.

That doesn't mean it won't happen, but it does suggest that the attackers will need a more ad hoc and fault tolerant architecture than Chord, something less efficient than Chord's brittle perfection but more workable in the real world.

Of course, the other day, the Internet was almost brought to its knees by an attack on the DNS root servers, and I never even noticed. So maybe the idea of attacking the net as a whole is harder than it sounds.