This is absolutely untrue. It’s a scare tactic that’s used to short-circuit rational debate about spending policies.
http://en.wikipedia.org/wiki/United_States_public_debt shows debt in absolute dollars and as a percentage of GDP for the US. We owed more money (as a percentage of GDP) in the 40’s and 50’s. Inflating the currency was not necessary to maintain that debt then and we’re clearly still here and making payments on it all.
From what I’ve seen, the US could pack on another $10-20 trillion in debt before anyone in the world financial community would be terribly concerned about our ability to repay. The real debate is not whether we can borrow/repay more debt, but whether we should.
For the OP, you can use the same Wikipedia article to get an idea of who owns US treasury bonds. One of the things to note is that about 3% of US debt (7% of the 44% held by foreign owners) is held by the UK. There are economic reasons regarding currency exchange and foreign trade for countries to have reserves in the currencies of other countries.