Most of us have always thought China owned most of it, right? I've even said so myself, although I was just parroting what other people said, without really knowing the facts (I hate it when other people do that, so why did I do it myself?). Turns out that's pretty far from the truth. Just like Japan, we own most of our own debt domestically. So all the talk about "what if China calls in their markers" really doesn't mean a whole lot (not that it ever did; that's not something they're likely to do anyway).
http://globalpublicsquare.blogs.cnn.com/2011/07/21/who-owns-america-hint-its-not-china/
I know a lot of you won't like the source (CNN), so go ahead and verify it elsewhere. I'm pretty sure the numbers will check out.