There's already people serving time for USD-related crimes.
I have two questions:
1) Since BTC is decentralized and apparently subject to no law or taxation, how do you propose bringing a bitcoin thief to "justice" and under whose law?
2) If bitcoin miners and traders are not paying any taxes on their bitcoin related activity, why are they entitled to the protection of whatever society they're evading taxation from?
Gareth's probably right on Fraud, but the most valuable thing you lose is credibility. Even when everyone is anonymous, you don't poo where you eat so to speak. That's bad for business, and once people realize that some people make bank by running Gox or Bitinstant by being honest and open, and they are losing out by ripping people off little by little, then there's more of an incentive to be good and not evil. Unless evil is just more fun, in which case, you need to be so smart about your evil, you might as well be legitimate.