$11,046,247,657,049.48 (According to US Treasury Direct, 3/26/09)
The mounting US National debt, growing by billions every day, has recently topped the $11 trillion mark. If denominated in $1 bills, the cash would stack as high as the tallest building in the world, the 2683.7 foot Burj Dubai skyscraper… 1,474,918 times. At this height, it would create a block of bills with a base approximately twice the size of the Empire State Building's, which is just under the size of three American football fields.
If consolidated into a single stack of $1 bills, it would measure about 749,666 miles, which is enough to reach from the earth to the moon twice (at perigee), with a few billion dollars left to spare. If the amount was laid out, the area of the $1 bills would cover the state of Rhode Island three times over, and in $100 bills the amount would carpet about 3/4 the area of Washington DC.
It is also interesting to note that this number is approximately 13 times the amount of US currency in circulation, according to the Treasury bulletin, which lists the amount at $853.6 billion as of December 31, 2008.