For all the annoyance of being crammed into an aluminum tube at 35,000 feet with a bunch of strangers, air travel has offered one benefit: the ability to tell bosses and colleagues, "I'll be on a flight, so you won't be able to reach me."

So much for that excuse.

Wireless Internet service is starting to spread among airlines in the United States - Delta and American have installed it on more than a dozen planes each, and several other carriers are planning to test it.

For the airlines, always desperate for new sources of revenue, offering the service - about $10 for three hours and more for longer flights - was an easy call. And many passengers will cheer the development as an end to Web withdrawal.

But this new frill is hardly as benign as a bag of pretzels. It may be a new source of tension between passengers on packed planes. A flight attendants' union has even expressed concern that terrorists could use it to plot attacks.

And there is the inescapable fact that one of the last places on earth to get away from it all can now be turned into a mobile office.

Brent Bigler, a financial planner living in Los Angeles, said he paid the $12.95 fee on a recent American Airlines flight to New York, and spent several hours reading e-mail and searching the Internet. When his plane was delayed, he was able to reach a friend to say he would be late for dinner.

Even so, Bigler said he worried about the downside.

"This could be the same thing as what happened with cellphones and BlackBerrys," he said. "Once it's cheap and ubiquitous, employers might expect employees to participate. I may feel guilty if it were a Monday and I napped or read and didn't use the Internet to do work."

Airline executives said they were aware that the new service had the potential to raise issues beyond the bottom line.

"We want to be respectful of the fact that an airplane is a public place," said Ranjan Goswami, director of product development at Delta. "You're in close intimacy with other passengers and the cabin crew."

Delta has told its flight attendants to treat overly enthusiastic users of Wi-Fi - who might, say, forget to mute the volume on YouTube videos of skateboarding dogs - like people who imbibe too much. In other words, cut them off if they start bothering others around them.

"It's just like alcohol," Goswami said. "The flight attendants understand how to interact with that."

But the Association of Flight Attendants, which represents 55,000 employees at 20 airlines, though not Delta, views Wi-Fi as a potential threat to flight attendants' ability to keep order in the cabin, said Corey Caldwell, a union spokeswoman.

"Our duties involve securing the safety of the cabin, not acting as censor police," Caldwell said. "It just adds another layer of duties inside the cabin, which take away from the main requirement that flight attendants are on board for."

Caldwell said the flight attendants' union also feared that terrorists plotting a scheme on a plane could use Wi-Fi to communicate with one another on board and with conspirators on the ground.

"Right now, their ability to do that on board is limited," she said. "But we can see an instance in which this becomes a potential threat."

The Federal Aviation Administration bans the use of cellphones aboard planes because they may interfere with a jet's navigation system. But Wi-Fi, as most technophiles know, offers a way around that ban, since the wireless connections can be used to tap into Skype and other programs that offer telephone service via a computer.

Clarel Thevenot, vice president for sales at Xtellus of Jersey City, said that during a flight from Stockholm he donned a headset with a microphone to call a friend in Paris. "I made the call brief and pretty much said, 'I'm at 35,000 feet and I'm calling you,' " Thevenot said.

Both airlines are using Wi-Fi service provided by Aircell. For now, American is offering its service on 15 Boeing 767 jets, said September Wade, a spokeswoman. If the test is successful, American will consider offering the service on its entire domestic fleet, but it has not decided yet whether to do so.

On Delta, service is $9.95 for a flight of three hours or less, $12.95 for a longer flight. American-based carriers do not yet offer the service on their international flights, although Delta is exploring it.

If all 150 passengers on a typical domestic flight were to buy three hours of time, that would mean an extra $1,500 or so in revenue per trip - equal to selling several extra seats per flight. Delta said its service was too new to accurately gauge its popularity, and American would not say how many travelers were using the service.

By offering the service, airlines in the United States are catching up to many foreign carriers, like Lufthansa, which has offered the service for the past several years.

Travelers who have used it say the service works well for video clip sites like YouTube, although it isn't quite fast enough for streaming live events or television programs. They say, however, there is enough bandwidth to download a TV show from iTunes and watch it afterward.

"The name of the game is to give customers choices, and let them vote for their own desire," said Goswami of Delta, which plans to have Wi-Fi available on 330 planes by the end of 2009.

Goswami said the airline will keep track of how customers use the Wi-Fi technology, then decide whether to set limits on how customers can use the Internet. Airlines can and do block access to pornography sites, for example, and Delta, like American, is blocking access to sites that offer Internet voice services.

"A lot of it will be self-policing," Goswami said. "If you're not aware of it, your seatmate will make you aware."

But rather than fighting with their seatmates, more travelers will probably be wrestling with themselves about whether to use the service. After all, guilt is common in today's pressure-filled workplaces, said Gayle Porter, professor of management at Rutgers University in Camden, New Jersey

"We want excuses to relax instead of making a conscious decision to relax," Porter said. "We don't want to put ourselves in the position of saying, 'That's my choice.' "

Michael Gross, an author who lives in New York, said he, too, had mixed feelings about the availability of Wi-Fi on planes, although he has used it to send e-mail messages and write a post on his blog. "One of the great things about getting on an airplane," he said, "is that it's life out of time."