IT HAD to happen. Instant hotel computer checkout.

Computers already are providing airline schedules, sending replies by electronic mail to business and pleasure travelers' queries, making hotel or cruise reservations and booking complete vacation packages--among other services. Soon, if there are no bugs in the silicon circuits and no serious problems with consumer acceptance, tourists will be able to bypass the line at the hotel cashier's window, pay their bills and checkout without leaving their rooms.

American Express Travel Related Services Company Inc. and International Anasazi Inc. have announced a pilot program next month to test their new "Express Checkout Service" at the Doubletree Hotel in Scottsdale, Ariz. Officials say the system should be available to the lodging industry next year.

"Quick and easy checkout procedures are what travelers want most," according to Mack Koonce, a vice president at American Express. "Express Checkout is one way in which we can help reduce waiting time and make hotel departures a little easier." Here's how it works:

As in the case with hotel rooms currently equipped with computers, where guests can call up the latest stock market quotations, dining room menus or perhaps sightseeing information about the city, the Doubletree pilot program will involve installation of keyboards connected to TV sets in a limited number of the hotel's 206 rooms. Express Checkout can be used only if the guest has checked in with a credit card, and only with an American Express card during the October test. When a guest pushes a button on a remote control pad, all charges will be displayed on the TV screen with instructions for approving or disapproving the totals. If the guest disputes a charge, the system will respond and try to solve the problem. Card billing is handled as usual, without an extra fee for the service.

One important difference, explains Lisa Nolan, an Anasazi official, is that the company's new PIRCS service (Personal In-Room Communication System), of which Express Checkout is only a part, "uses a microprocessor in every room, not just a terminal to receive information." And each hotel will have its own in-house computer with cable-controlled video, rather than merely retrieving information via a telephone line. PIRCS will permit guests in different rooms to communicate --perhaps eventually including competing in video games.

PIRCS knows who you are and what room you are in, so there is no need to enter your name or number to access the system each time you use it. You just press a button and tune to the proper channel. You can even program your own wake-up calls. There will be no charge access to the system, when leased phone lines are not involved, and all basic services will be free. However, if you want extras like the Dow Jones figures and news reports from the wires, or need to connect with your office computer while on a business trip, PIRCS can give you access to a remote data base by plugging you (and the microprocessor) into the phone line--for an added charge. The entire system will be on line next month for the pilot project. The entire hotel is expected to be wired up by the end of the year.

Anasazi, a privately held corporation with headquarters in Phoenix, sells software and hardware, numbering among its clients Best Western International, Quality Inns and Days Inn.

About that hotel bill--if you don't like to argue with a computer, you can still march downstairs and talk to the assistant manager.

"TOUCH & GO": In yet another computer development aimed at the tourist market, Auburn University education professor Terry Countermine has developed a novel system to guide out-of-town visitors to hotels, restaurants and attractions that eliminates any need for them to learn how to operate a computer keyboard. And it automatically gives them a printout.

Countermine, who teaches students how to use microcomputers in business and education, designed his software program around a video screen which, when touched by a finger, generates an electronic impulse that activates a computer. His company, Microvations Inc., headquartered in Auburn, Ala., has installed the "Touch & Go" system in Alabama highway welcome centers, a convention center in Gulf Shores, Ala., hotels in Atlanta and Shreveport, La., and in several hotels and a welcome center in York, Pa. The machines also will be installed in welcome centers in Mississippi, Louisiana and Georgia, and inquiries have been received from other areas.

The operation in the Georgia welcome center is typical. When a tourist touches the screen, it displays a color map of the state divided into seven regions. When the user touches the region he wants to travel to, the screen then lists the cities. Finally, he touches the name of a city and the type of service sought there--lodging, restaurant or attraction--and the screen displays names and directions while the computer prints out a copy the visitor can slip into his pocket.

The computer printout may also include rates, advertisements and even money-saving coupons. Unfortunately, the listings it provides are not all-inclusive. Since each system sells for about $5,000, buyers seeking to recover their investment charge an advertising fee to list commercial establishments. (Parks and other nonprofit facilities are included free.)

NUCLEAR ATTRACTION: In the tourist business, even a nuclear accident can become an incentive--assuming the force is with you.

Though Three Mile Island Nuclear Station near Middletown, Pa., is no competition for major destinations like Hershey, Gettysburg or the Amish area, it has become one of central Pennsylvania's "most popular" attractions, according to the GPU Nuclear Corporation. Guests at nearby hotels often ask if they would be permitted to drive by the plant that made alarming headlines.

Before the TMI-2 accident in 1979, the only part of the generating station regularly open to the public was the Visitors Center on Rte. 441 across from the plant. But after what has become known as the nation's worst commercial nuclear accident, the corporation began giving in-plant tours to satisfy increased public interest, and then added drive-around bus tours.

More than 300,000 people have visited Three Mile Island since the accident and about 10 percent have taken the free tours. The tours provide information on how a nuclear power plant operates and explain how the cleanup of TMI-2 is progressing, says William Gross, Visitor Center manager. None of the tours has involved any instances of radiation exposure, the company says.

In-plant tours (minimum age is 13) take three hours, begin with a video tape at the center and include visits to the TMI-1 and TMI-2 control rooms. There are no age restrictions on the one-hour bus tours. The south end of the island is a semi-wilderness tract with a large herd of deer, waterfowl and a variety of other animal and plant life. A multisensory nature trail has been created especially for the visually handicapped. The company also provides transportation to that area.

To schedule specific tours, or for more information, phone the Visitor Center at 717-948-8829. The center, three miles south of Middletown is open from 10 a.m. to 5 p.m. daily.