The US is in a unique situation as the US dollar has been accepted as the exchange standard and as the Reserve Currency. So when it needs to borrow money, it can borrow US dollars and repay with US dollars. In addition, the US Government is the only one allowed by law to print US dollars. Because it is held as a reserve and used for exchange, the US can get away with financing its own debt with the costs being absorbed by everyone who holds the dollar (which is a far greater number than all other countries).
The Federal Reserve is a govenment entity that is part of the Department of the Treasury that handles both the printing of money and issuing of Treasury Bonds. Because of this the US can choose never to default.
That said when you get a treasury bond for $1,000,000 you buy it for $999,500 with a 6 month maturity date you expect that in 6 months you will have a bond worth $1m. But if the government decides during that 6 month period to create a signifigant quantity of dollars, such that the value of the dollar is diluted so $1,000,000 is only worth $999,450, it is not default but it is also not what you expected when you purchaces your T-Bill 6 months earlier.
If the US had to buy, say, gold to back its T-bills, let's look at the two values: a 6 month T-bill bought Feb 1 for $1m versus a T-bill that promised an amount of gold worth $1m on Feb 1 with a July 1 Maturity.
Format is Currency $1m Feb 1 2011 - Value of return on July 1 2011 in US$ vs had you kept your original currency
US Dollar - $940253.16
Gold - $887230.95
Euro - $952920.52
Canadian Dollar - $967767.47
Feb1 xch rates
Jul1 xch Rates
So actual default? No. But If I took a $1,000 loan from the bank and when it came due I only paid them $940, they are not going to just accept that as paid in full. But government effectively does just that.
This is changes in valuation not inflation.
US Inflation rate - 3.5588% (Per year)
Devaluation rate - 11.94% (Feb1-Jul12011)
To be fair QE is not the only downward pressure on the value of the dollar. We are currently reaping the result of overspending and policies that encourage wealth exodus. This is not the result of any one president, but goes back about 20 years, through several changes in leadership of all levels of government.
(Updated gold to reflect the same metrics as other currencies)
UPDATE CONSUMER CONFIDENCE CRASHES
The whole point is of this is that while technically and legally, there is no default the expectation is that the government will honor its debt. If they allow their currency to devalue, allow hyper-inflation to set in, and print money to pay debt, then they are not doing that. If it was just gold that was going up then it would not hold water. But look at all of the currencies listed. It shows consistent losses by the dollar. Roughly around 6-8% almost across the board over 6 months. That is effective default even if it is not technically default.