See also my comment on this DailyKos diary: Two signs that something is seriously wrong

Most of the international shipments use letters of credit to guarantee payment. It is one of the oldest and safest form of short-term credit and it is vital for international trade. As early as October, it has been reported that sellers started to refuse buyers' letters of credit because they didn't trust tha banks issueing them anymore.

naked capitalism: International Trade Seizing Up Due to Banking Crisis

Letters of credit. issued by banks, assure payment. They can also serve to finance the shipment (ie, fund the inventory while it is in transit).

Not only are banks now leery of lending to each other for much longer than overnight, they are also starting to refuse to honor letters of credit from other banks. From the above-mentioned reader:

At the end of the day, if every counterparty is bad then you don't have a market and you don't have an economy. I spoke to another friend of mine this afternoon, whose father has been in the shipping business forever. Pristine credit rating, rock solid balance sheet. He says if he takes his BNP Paribas letter of credit to Citi today for short term funding for his vessels, they won't give it to him. That means he can't ship goods, which means that within the next 2 weeks, physical shortages of commodities begins to show up.

We spoke later in the evening and said he had heard of another instance of a trade transaction failing, different parties entirely, this a shipment of coal, again due to the unwillingness of the seller's bank to accept an LC from the buyer.

Confirmation comes from the Financial Post, "Grain piles up in ports" (hat tip reader Vox Sanus):

The credit crisis is spilling over into the grain industry as international buyers find themselves unable to come up with payment, forcing sellers to shoulder often substantial losses.

Before cargoes can be loaded at port, buyers typically must produce proof they are good for the money. But more deals are falling through as sellers decide they don't trust the financial institution named in the buyer's letter of credit, analysts said.

"There's all kinds of stuff stacked up on docks right now that can't be shipped because people can't get letters of credit," said Bill Gary, president of Commodity Information Systems in Oklahoma City. "The problem is not demand, and it's not supply because we have plenty of supply. It's finding anyone who can come up with the credit to buy."

That probably explains the dramatic fall in the Baltic Dry Index. If it expands, we could see the international trade coming to a halt and shortages (of food, raw materials, fuel...) taking place with ugly social and political consequences...