A Wall Street Journal indicates that the US Geological Survey has recently concluded that US coal reserves may not be as plentiful as once thought. The total amount of coal resources is unquestioned, but USGS now estimates that under 6% of resources are economically recoverable.
Coal provides nearly half of US energy needs, and US production is 1.15 billion short tons of coal; this makes the US second in the world behind China and ahead of India (2.80 and 0.53 billion tons respectively). Coal production is expected to drop 5-10% this year, probably most of which is due to a lull in demand due to the recession.
USGS estimates that there are 201 billion short tons of coal in Wyoming's Powder River, only 77 billion of which are physically recoverable. Coal from basin currently sells for $8.50 a ton. At $10.50 a ton, only 6% of the coal could be profitably extracted (4.6 billion tons). At $60 a ton, as much as 47% (36.2 billion tons) could be extracted. But at that price, coal would be hard pressed to compete with other fuels - and that's a good thing.
In 2007, USGS estimated that the US had 500 billion tons of coal resources, 267 billion of which were economically recoverable (they assumed mining companies would need to see an 8% return for the example above). 6% of that number is 30 billion tons - enough for under 30 years worth of estimated 2007 coal production. Coal prices will rise, unless the US imports.