Good news: iPad could make AT&T's service even slower

“AT&T seems to be convinced that most of the time users will be connected to Wi-Fi,” said Craig Moffett, a Sanford C. Bernstein & Co. analyst who rates the shares “market perform.” “That’s a pretty big stretch, given it’s a new device nobody’s used before.”

An iPad on AT&T’s third-generation network will consume about two-thirds as much network capacity as an iPhone, according to independent wireless industry analyst Chetan Sharma. That could be the equivalent of 1.7 million additional iPhones hitting AT&T’s network this year, assuming Apple sells the 2.7 million of the devices that Piper Jaffray & Co.’s Chris Larsen forecasts.