The constraints on financing the current account deficit are likely to come from "foreign investors' fears" of holding too large a share of their investment portfolios in U.S. stocks and bonds, Greenspan said.

This change in mind-set could already be under way, he suggested.

"Concentration and other risks in holding dollar balances seem to have become a consideration at least for some investors, Greenspan said.

I think I started discussing this problem (more in terms of the federal budget deficit than in terms of the trade issues though) here over two years ago. It's gotten worse since then with no let up, and Greenspan doesn't seem to have any ideas on how to fix it.