Until about 2007, the highest outhouse in the United States sat 14,494 feet (4,417 meters) above sea level, perched on the summit of California's Mount Whitney [Source: Barringer]. It's a tricky business hauling waste down from such great heights, however, and in recent years, the park service has pared back by eliminating privies from a number of high-altitude destinations.
These days, hikers who pick up forest service permits to hike the Whitney Trail are issued sanitation kits for packing their human waste back out with them once they complete their trek. In lower elevations, campers can pack a small camp shovel and bury their waste. At high altitudes, however, the ground can be so rocky or frozen, and the ecology so fragile that burying your waste is out of the question.
If images of oozing Ziploc bags have your stomach roiling, you'll be pleased to know that new products like the Wag Bag (Waste Alleviation and Gelling Bags) offer an easy, biodegradable, odor neutralizing "toilet in a bag" solution.