(FORTUNE Magazine) – For Michael J. Fagel, a disaster-recovery specialist, wireless data service came of age in the rubble of the World Trade Center. While working in the chaos of ground zero just days after the towers fell, he says, "no cellphones worked, land lines were inoperative, and I needed answers about hazardous materials. There were no books or data, because we couldn't carry them on the site."

But Fagel was carrying a Palm VIIx wireless handheld PDA (personal digital assistant), which allowed him to send text messages to colleagues in Washington, D.C., and Chicago on the spot. "I would pose questions on e-mail, and in ten or 20 minutes friends would come back with answers," he says. As other wireless services were restored, Fagel added more tools: a Nextel voice pager, an OmniSky pager, and a Nokia cellphone. "Sometimes I had all four devices active at once, sending and receiving messages and gathering data from every source," he says. "They were worth their weight in gold."

Few executives will ever encounter such extreme conditions, of course, but the lesson is clear: Wireless data devices can be quite valuable for business communications. Thanks to the popularity of electronic mail and recent improvements in the reliability and availability of wireless data networks, handheld data devices (see three of the best models at right) are becoming standard equipment for mobile workers and for executives who are frequently away from their desks. Their small size and growing capabilities make them particularly attractive now that carrying laptop computers onto airplanes has become a hassle.

So far, more than 13,000 corporations have equipped their mobile executives with RIM BlackBerry two-way e-mail devices, letting them tap into corporate e-mail systems from nearly anywhere in North America. RIM's success has caught the interest of companies from Palm to Microsoft. Microsoft is preparing software to power wireless handheld devices that combine e-mail and voice applications.

Many companies like their employees to use PDAs because they can boost productivity away from the office. At the Pepsi-Cola bottling company in Buffalo, for example, the use of wireless handhelds has cut 90 minutes from sales reps' average daily route time and has virtually eliminated paperwork for thousands of sales calls. Employees like them too. Jeff Shield, a vice president and sales manager at Siebel Systems in San Mateo, Calif., who is on the road several days a week, receives about 100 e-mails a day. For urgent matters he relies on a cellphone, but for routine responses he uses the BlackBerry (although he says he plans to switch to a Palm). "It enables me to use dead time throughout the day," he says.

Indeed, as PDAs proliferate, employees are pressuring their companies to integrate them into the corporate network. Such was the case at the Clos du Val winery in Napa, Calif., where senior managers had begun buying Palms with wireless e-mail capabilities. The devices spread to the sales department, where they were used to coordinate calendars and keep wine-tasting logbooks. Clos du Val is now working to wirelessly deliver real-time inventory lists and purchase histories to its sales force and distributors.

But companies have concerns about PDAs too. Information managers are justifiably alarmed at the idea of employees running around with sensitive corporate data packed into a small device that is easily lost or stolen, and they worry about viruses and the vulnerability of transmitted messages. Because of the wide array of new devices and data carriers, businesses are also struggling to determine how to manage both support and costs. And even though wireless data services proved their worth after the September attacks when conventional communications lines were down, in general wireless connections are more finicky than wired ones, as any cellphone user knows.

But these are still early days. If you buy the premise that the coming rollout of third-generation phone networks will allow handheld devices to send and receive voice calls, data files, e-mail, instant messages, and even video teleconferencing signals, then hundreds of millions of wireless communicators will eventually be in use worldwide. And it's a safe bet that one of them will be in your pocket.