Most tablets still are sold with only Wi-Fi. For Apple's iPad, by far the dominant tablet, no more than half of the units sold are equipped with cellular, said IDC analyst Bob O'Donnell. Buying a 3G or 4G model adds cost both for the hardware purchase and for the service plan.

But AT&T is happy with the sales rate of cellular-equipped tablets and the adoption of data plans for them, said Glenn Lurie, president of emerging devices, in an interview at CTIA Wireless. And the carrier is talking with manufacturers about making their product lines consistently equipped for 3G or 4G, he said.

"All devices should have all capabilities built in from the beginning," Lurie said. One reason that hasn't become a reality is that the additional hardware has been too expensive in the past, he said. "The ecosystem was a little out of balance."

Now, the cost of a 3G HSPA+ radio to device makers is only about $30 on an average, Lurie said. (However, the difference for an LTE radio is still around $60 to $70, he said.) As the cost premium gets thinner, he expects tablet lineups to consolidate on a single design with both cellular and Wi-Fi.

One tablet sold by AT&T has already gone that way, Lurie said: The Pantech Element, a combination Wi-Fi and LTE product that AT&T offered for $399, and which has already sold out, according to Lurie. But there are other such products in the planning stages, he said.