Let's imagine that the entire staff of MtGox were to be arrested tomorrow. Their homes burned, the earth salted, etc. So no more Bitcoin to USD trades at MtGox.
But what about your neighbor Bob's place? You buy him dinner, and the next day a string of bits comes your way via Tor (or somesuch.) There is no need to utter the word "Bitcoin" during said dinner. A wink, a nudge, etc. will suffice. The next day, Bob does the same with his neighbor Charlie. And so on.
And, naturally, if Bob were a traitor, he could rat you out to the Gestapo. But he could just as easily rat you out for [insert extremely popular but highly-illegal-in-the-USA activity here.] No digital cocaine required - the analog kind will do. And if you haven't any under your bed, Bob will helpfully leave a kilo in your toilet tank.
Most people have already had plenty of practice exchanging highly-illegal strings of bits. Now they will just need to learn to give each other "free" stuff as well.
Bitcoin hasn't reached the critical mass needed for this kind of thing, but there is no reason why it could not.
Consider hawala (http://en.wikipedia.org/wiki/Hawala) which is doing quite well despite draconian bans. On which currencies do hawala transactions depend? All of them and none of them.