Lots of assets I should think. This article explains the point I think (although the figures are way out of day and the situation now much worse):-

http://www.safehaven.com/article/2292/when-our-dollars-come-marching-home

Situation is similar in the UK as we've been running persistant trade deficits for years also. However, I think the pounds, being less sort after for reserves have been to some extent gradually repatriated over the years with much real estate and stocks in foreign hands already.