Don't deposit any money on bitfinex and if you have coins there, withdraw them - better safe than sorry.
That would be the best possible way for people to react, but I am fairly sure that just a tiny fraction of them will actually do so. Most people will only learn when things cost them a heavy load of money, but then it's already too late to act. I am not trying to scare people off, but the Mtgox smell starts to become stronger and stronger.
Well yes, that's by definition how it works if Bitfinex is insolvent - it basically becomes a ponzi scheme. Those few who get out get out, the rest is inevitably trapped. The USD trapped is always going to be trapped, by getting out you're only changing >who< gets trapped. The majority would inevitably end up trapped.