Thanks for posting this, I was looking for it for a few days. I think this letter is extremely significant. Bringing down some big bridges that span harbors and ports could also disrupt shipping - there would be big pieces of bridge in the way for weeks. I could see the Baltimore and Boston tunnels being targets for the same reason.
Also, we should be ready for direct attacks on, or from, cargo ships -- remember the story earlier this week about the Turkish man that was found on a ship in Europe hiding in a container. Perhaps that was a trial run for an attack involving terrorists shipping themselves into U.S. ports on cargo ships and either setting off bombs or shooting missiles from the ships. This would have the same effect on our shipping that 9/11 had on the airlines - everyone would be afraid to get on a ship or ship their goods for weeks.