Interesting academic paper by Dorit Ron and Adi Shamir from the CompSci department at The Weizmann Institute of Science in Israel on how the Dread Pirate Roberts protected his Bitcoins.
Most interesting is that the Bitcoin community could have prevented the transfer of the F.B.I cised Bitcoins by not verifying the chain block:
An interesting comment we would like to make is that the notion of seizing bitcoins from a suspect’s laptop is much trickier than the notion of seizing cash from a suspect’s safe, even if all the necessary keys are found by the FBI in both cases. In the case of cash, once the money is hauled away, it is no longer available to the suspect. However, let us assume that the Bitcoin community had noticed the unusual activity, and had refused to pick up these FBI-initiated transactions for verication as part of the official block chain. In this case, it would not help the FBI that they set up the new account and had initiated those transfers - their holdings would not be recognized as valid, and thus they would not be able to exchange or auction them off. In addition, they could still be used by either the suspect or by any one of his accomplices who happens to know the secret key! Even if the community had been late in recognizing these events and some miners would have picked up those transactions in the meantime, a 51% majority of the computing power available to miners could have forked the block chain just before these transactions, and grown a longer side chain which would invalidate all the blocks that contain the FBI transactions. However, by now it is probably too dicult to take such measures, and the seized bitcoins are no longer usable by DPR.
Dorit Ron and Adi Shamir in How Did Dread Pirate Roberts Acquire and Protect His Bitcoin Wealth?