SOMEWHERE OVER THE MIDWESTERN UNITED STATES -- Not very long ago, the idea of using a computer in an airplane seat would have been ludicrous. Even after huge mainframes gave way to personal computers, nobody in their right mind thought of carrying around a heavy monitor, CPU, keyboard and accessories.


There was a good reason they were called "desktop" computers: They belonged on top of a desk.


But then came the first portables, charmingly dubbed "luggables," and then machines that truly were portable: laptops. Today, the laptop and its progeny, the notebooks and subnotebooks, are near-standard parts of business travelers' luggage.


As you go through any airport, you will see countless men and women toting the distinctive soft-sided cases that hold today's portable PCs. Millions have been sold in just the past couple of years, transforming what once was a high-tech status symbol into a common, indispensable traveling tool.


Big advances in power and equally sharp declines in price have allowed users to carry their electronic work (and play) with them and set up shop in a hotel room, an airport waiting lounge or at 30,000 feet.


But for all their popularity, portable laptop PCs still have their drawbacks, as any regular user knows. These problems are not so much flaws in the machines themselves, but inconveniences wrought by a world that still is not quite ready for the ubiquitous portable PC.


For instance: I am typing this column in a cramped airline seat, and my laptop's screen is tilted at a bizarre angle to cope with the well-reclined seat in front of me. I just had to change batteries so that I could continue to work. And, once I have finished, there is no way for me to transmit my work from the airplane back to the office.


Things are not that much better on the ground. For some reason, most hotel rooms are not designed to allow guests easy access to power outlets -- a real drawback when you want to plug in a laptop to run off AC power or to recharge your ever-draining batteries.


Batteries, of course, are the greatest drawback of portable PCs -- most last only a couple of hours or so (a lot less on high-powered color machines). There are some interesting new extended-life batteries that provide a few more hours. But they weigh more or take up more space.


The fact is that battery technology has badly lagged behind portable computing technology. The computing world will beat a path to the door of the first company to come up with a truly long-life portable PC battery -- one that lasts, say, 20 or 30 hours or more. Then there are the telecommunications hassles. Some hotels still have not caught onto the fact that many travelers want to plug their computer modems into their room phones to tap into on-line networks or to exchange information and e-mail with their offices. While some hotels offer "data" phone extension ports on their telephones to make it simple to plug a computer into a dial-up telecommunications network, others force the traveling computer user to unplug the phone from the wall and practically rewire the room.


And a few hotels just do not get it at all. They still use antiquated phone systems that do not have modular jacks into which guests can plug a modem, or they have newfangled phone systems that use digital connections that can fry a computer's modem.


Technology, as always, is likely to lend a hand. Beyond better batteries is the potential for powerful wireless computer communications that will make the portable computer an even more impressive tool -- able quickly to connect into data networks from anywhere.


Not long ago, that idea was science fiction. But then, so was the idea of using a computer at 30,000 feet.