Who's buying US debt?

In a speech tonight at Georgetown University, New York Mayor Michael Bloomberg -- a savvy financial guy -- had a few things to say about the current crisis.
Here is one that caught my eye:
It's not clear who's going to be buying our debt. It may very well be that the next wave is going to come back and bite us.
And if the U.S. has to raise rates in middle of this ... let's just say it won't be pleasant.
More here.
http://paul.kedrosky.com/archives/2008/09/17/bloomberg_says.html