Rebuttal was already posted:
The Feds recently subpoened Mt.Gox to force them to retain trading records, so the chain of ownership will be known to law enforcement.
You think it's a given that MtGox has detailed, accurate and long term trading records? From a company that managed to lose all but 2000BTC out of 750,000BTC (if the story is true) before they noticed and pulled the plug?