It gets worse: Suppose a fraudster swindles someone else out of their money. The victim can go to the authorities with the fraudster's Bitcoin address. The authorities can force Mt. Gox to halt and flag any attempt to trade Bitcoins that, according to the transaction history, were once at the Bitcoin address that was used by the fraudster.
If the fraudster then spends his Bitcoins, and they eventually end up in your hands, and then you take the funds to Mt. Gox, you can end up getting charged with "receiving stolen property", even if the fraudster is never found.
The more likely scenario, however, is that the state attacks Bitcoin by shutting down Mt. Gox and similar services, making it next to impossible to convert Bitcoin into other currencies.