Roustabouts, weatherpeople and Air Force pilots 

As if to underscore Apple (AAPL) CFO Peter Oppenheimer's claim that "nearly all" Fortune 500 companies "approve and support" iPhones on their networks, three major purchase orders came to light this week:

Halliburton announced that over the next year the oil services company will be "transitioning" from Research in Motion's (RIMM) BlackBerry platform to "smartphone technology via the iPhone." A spokesperson told AppleInsider that about 4,500 Halliburton employees currently carry BlackBerries.
NOAA, the National Atmospheric and Oceanic Administration, is also dumping its BlackBerries, according The Loop's Jim Dalrymple, and replacing them with iPhones and iPads. The agency's 2011 budget was $5.6 billion.
The U.S. Air Force may buy as many as 18,000 iPads to lighten the load of flight crews, according to a Bloomberg report. The purchase order would be one of the military's biggest -- if not the biggest -- order of computer tablets to date.