Adam Tooze at the NYT has the very best piece I have seen on this question. You do need to read the whole thing, but here is part of the opening bit:
For the second time this century, the world is facing an acute shortage of dollar funding. This is a big problem: An enormous amount of global financial activity depends on the use of the dollar. If we are to contain the fallout from the crisis, America’s central bank must act as a lender of last resort not just to America’s financial system but also to the entire world’s.