(':kewl:')
It appears the U.S will have a stamp depicting a cave at long last, probably issued in May. It is included in a sheet of 40 stamps called Wonders of America, Land of Superlatives. You can view the sheet of stamps, and the Mammoth Cave
stamp, at the following link (scroll down about half-way through the site).
http://www.usps.com/communications/news ... 05_054.htm
The stamp shows a group of columns. It doesn't exactly cry out "Mammoth Cave" but at least it recognizes our most well-known cave.
For the speleophilatelic purist, yes, there was an earlier imprinted post card that did feature Carlsbad, which is probably no longer available since rates have gone up.