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?
I doubt that this would happen. The Bitcoins would not have the same address. They are not the same Bitcoins.
Maybe Bitcoins obtained straight from a stolen source are one thing, but Bitcoins that have transferred MANY times with differing amounts over a year? What basis would that be? They do not do that with cash or currency.