You know it's time to get nervous when Warren Buffett starts warning about our trade deficit and says

Through the spring of 2002, I had lived nearly 72 years without purchasing a foreign currency. Since then Berkshire has made significant investments in—and today holds—several currencies. I won't give you particulars; in fact, it is largely irrelevant which currencies they are. What does matter is the underlying point: To hold other currencies is to believe that the dollar will decline.

Both as an American and as an investor, I actually hope these commitments prove to be a mistake. Any profits Berkshire might make from currency trading would pale against the losses the company and our shareholders, in other aspects of their lives, would incur from a plunging dollar.

But as head of Berkshire Hathaway, I am in charge of investing its money in ways that make sense. And my reason for finally putting my money where my mouth has been so long is that our trade deficit has greatly worsened, to the point that our country's "net worth," so to speak, is now being transferred abroad at an alarming rate.