This is very troublesome. The US is actually much worse off financially than Greece. When people discuss the Greek national debt, what you see it what you get. When people talk about the US national debt, they are leaving out all the states, cities, etc. (The real number is somewhere north of $60 trillion.) What keeps us afloat is the fact that our currency is the global currency of choice for world trade and the reserve currency of choice for most central banks. Governments, businesses, and individuals all want/need to keep dollars in order to execute transactions, hedge against currency fluctuations, or just stuff in their sock drawer for a rainy day (note: that is not where I keep my stash, so lay off my socks). If the US dollar is no longer seen as the global reserve currency, it will collapse.
The Russians are actually making a move against the petrodollar. It appears that they are quite serious about their de-dollarization strategy. The largest natural gas producer on the planet, Gazprom, has signed agreements with some of their biggest customers to switch payments for natural gas from U.S. dollars to euros.