Does Heroku protect individual sites from DoS / DDoS attacks?

Heroku is, it seems, under a DDoS attack right now, which is causing intermittent availability issues across the site manifesting themselves on (of course!) my app.

I have seen a number of these kinds of attacks recently, including the huge DDoS attack on Register.com a few months ago.

My question is, what were to happen if attackers zero'ed in on one of Heroku's clients?

Does Heroku protect individual apps from DoS and DDoS attacks?