That's probably the only PDA you can order with a night-vision-goggles filter. A company called Elbit Systems manufactures ruggedized PDAs and handheld computers for the U.S. Army like the aluminum and waterproof RPDA-57, above. Check out the super-chunky battery pack that holds ten AA batteries, which are presumably easier to come by in the field than the D-batteries you'd expect such an enclosure to hold.
Admittedly I don't know much about military field-usage of computers, but I find it astonishing that they'd design anything to use a stylus--I'd lose one of those in my office, never mind in a field laced with gunfire. Elbit's ETC MK IV handheld computer (below) at least has some keys attached as well, while the Tacter (bottom) seems to make the most sense as it's got a flip-down keyboard.
Those curious about military-inspired form factors can peruse Elbit's full line of products here, but be warned, they definitely don't make it easy to navigate their site.