If someone steals a $100 bill from you and you have the serial number. If it is ever found, it will be returned to you with the person/company which had it bearing the loss.

Same is probably true for Bitcoins stolen from Mt. Gox.

You have a Bitcoin, but are you sure you really own it? Are all of them now suspect? Or, are all of the coins generic within that system with no identifiers?

As the worm turns....