That is an interesting thought, but may be difficult in practice.
A theft occurs coins are transferred...
> Mt Gox -> Wallet X
Some time goes by and the coins are moved around
> Wallet X -> Wallet Y -> Wallet Z
Finally, the coin is moved to somewhere we can track, like a well regulated exchange.
Wallet Z -> A regulated exchange
FBI would have to backtrack from that point. There might be some significant cost for each step.
I also wonder how well coin mixing services work. For huge volumes like this, I can't imagine them being effective. But for smaller amounts, maybe over time, they may prevent that backtrace from being feasible.