Ahem. Please don't attribute Goldman's success to trading. Goldman Sachs recent "success" is largely due to them adroitly navigating the political waters of the bailout, without which i suspect they would be in serious trouble.
As for machine trading or stat-arb, high freq, whatever...i don't think it is any better than a random walk. Risk was (is?) just being hidden somewhere and lied about.
No. You need to find enough of an arbitrage opportunity to overcome the friction. But once you found it, it's a risk-free profit. (You can't do too complex an atomic transaction on the financial markets; but most arbitrage deals take much less than a day --- even less than a few minutes, so your window of exposure is quite short.)
As for machine trading or stat-arb, high freq, whatever...i don't think it is any better than a random walk. Risk was (is?) just being hidden somewhere and lied about.