It's finally happened: After years of go-go growth, the number of people signing up for cell phone service in the United States is finally slowing.

The shift could spell good news for consumers as carriers turn up the marketing heat, said Craig Moffett, a senior analyst at Bernstein Research and author of a report on the trend.

"When operators have no choice but to try to take customers away from each other, they have a natural inclination to sharpen the pencils and make the best offer they can," he said.

The wireless industry added 23 percent fewer new subscribers in this year's first quarter than in last year's.

The overall growth rate — how fast the total is growing — dropped to 7.9 percent from 11.5 percent, and will "slow further from here," the report said.

Moffett said carriers are victims of their own success. "The vision of every adult in America having a cell phone is a great aspiration until you get there. Then it raises the obvious question: Now what?"

For years, wireless was a story of heady growth. As of year-end 1995, the U.S. had 33.8 million subscribers. By December 2007 it had 255.4 million, a seven-fold increase.

About 83 percent of U.S. consumers have at least one cell phone.

By 2010, the number will rise to 89 percent, Bernstein predicted.

At that level, Moffett said, everybody who wants a cell phone will already have one.

For carriers, wireless analyst Charles Golvin of Forrester said, it's now about "stealing your competitors' customers."

For now, AT&T and Verizon continue to add a lot of customers — more than 1 million each in the first quarter. But Moffett said that's only because they can feed on Sprint Nextel, which is struggling with operational problems. Customers are deserting Sprint by the thousands, providing easy pickings for rivals, he said.

"If Sprint can right the ship, then subscriber numbers for the rest of the industry are necessarily going to have to take a leg down," he said.

Carriers are pinning their hopes on wireless data, which is exploding as Web-enabled wireless devices like the Apple iPhone start flooding the market.

At AT&T, the sole U.S. iPhone distributor, data revenue is growing by more than 50 percent every quarter.

Moffett said that's not enough to replace lost growth of wireless voice customers, who still account for 87 percent of revenue growth at Verizon and AT&T.

Mark Collins, an AT&T vice president, allows that it's tougher to recruit new subscribers. On the upside, he said, there are plenty of revenue opportunities. "That's why we've been investing like we have for the past three years" in high-speed wireless data and advanced video services.

And Sprint said its future is bright.

"We believe the future of wireless is data, and that's our sweet spot," said spokeswoman Leigh Horner.

Sprint has teamed with Google and other backers, including wireless visionary Craig McCaw, to build an advanced wireless data network across the U.S.