It could take years before the block chain is long enough to dissuade a double-spend attempt for a transaction that large, where you have a single party single-

It could take years before the block chain is long enough to dissuade a double-spend attempt for a transaction that large, where you have a single party single-handedly accounting for a significant fraction of spending on the network. So for a single $40b transaction, perhaps bitcoin is infeasible (or at least must be combined with traditional mechanisms like reputation, courts, etc.), but for a million independent $40k transactions, they can all be confirmed in a matter of hours for only a few hundred thousand dollars worth of computation, total.