Sorry if this was discussed, but in USA if you buy stolen property and you are caught with it you have to return it to the original owner at your loss.
Now, lets say someone did steal the Mt Gox. Bitcoins, and then you bought some of them unknowingly.
If they could trace back through the blockchain that those bitcoins were indeed stolen Mt. Gox. coins, wouldn't they be able to take them back from you?
Stolen money circulate in the economy, why should bitcoin behave differently?
As long as you did the trade in good faith you should keep your bitcoins even if they were stolen by the seller.
The thief should be punished, not you.