The US government (via the Fed, etc.) is the only legal authority that can create dollars. As such, there is no conceivable circumstance where it cannot pay debts denominated in dollars.

As such, it cannot default due to lack of dollars.

On the other hand, any debt or other financial obligation taken on by the US government is (AFAIK) legally the same as any other. If one says that the US has to pay bonds on time, then it has to pay Social Security and salaries on time, too. Treasury Bills aren't special in that regard. We can't "selectively default" by paying off bonds but not other things.

When there are periodic scares about the government shutting down because it doesn't have money, its not due to the US being unable to create or raise dollars. It's due to the budget rules and insane Teabaggers wanting to burn the federal government down.

Greece is different because it cannot make its own currency. At the moment...

But you knew all that. ;-)

HTH.

Cheers,
Scott.

Hint (11 page .pdf):

Zimbabwe’s economic crisis and subsequent hyperinflation were preceded by several years of economic decline and mounting public debt. Weakening began in 1999, coinciding with periods of drought that adversely affected the agriculturally dependent nation. External debt as a share of GDP increased to 119 percent in 2008 from 11 percent in 1980. Land reallocation in 2000 and 2001, which redistributed large agricultural tracts, depressed commercial farming output. Output fell 50 percent between 2000 and 2009, led by a decline in the country’s major foreign-exchange cash crop, tobacco, which slid 64 percent in 2008 from 2000 levels (Chart 3). Commercial production of maize, the national staple, dropped 76 percent during the same time (FAOSTAT Database 2011).

Uncontrolled government spending accompanied the weak economy. In 1997, authorities approved unbudgeted expenditures, amounting to almost 3 percent of GDP, for bonuses to approximately 60,000 independence war veterans. Efforts to cover the payment with tax increases failed after trade-union-led protests, prompting the government to begin monetization (printing additional money to “pay” for the expenditure). In 1998, the government spent another significant share of gross national product (GNP) for its involvement in Congo’s civil war. Additionally, authorities faced debt obligations to the IMF. In 2006, Zimbabwe still had substantial overdue obligations to the IMF’s Poverty Reduction and Growth Facility and Exogenous Shocks Facility Trust, totaling about US$119 million. 6 These funds were intended to foster development and reduce poverty.

The dire economic conditions prompted a wave of emigration to neighboring countries, contributing to a population and labor force decline beginning in 2003 (Chart 4). Zimbabwe emigration totaled 761,226, about 6 percent of the population in 2005. This number increased to 1.25 million in 2010, representing 9.9 percent of the population (World Bank 2008 and 2011). With a shrinking tax base and revenue that could not support expenditures and obligations, the government printed yet more money. Currency lost value at exponential rates amid an imbalance between economic output and the increasing money supply (Chart 5).

Just like the USA!!11 Amirite? :-/

Cheers,
Scott.