AT&T has been pleased with the iPhone, despite their convoluted issues with initial activation. And, sales if the iPhone were very strong at least, Apple thought they were, and assumed that subscribers would soon follow sales. For all its merit, however, the iPhone isn't doing as well as Apple would like.

As a result, their stock price has dropped and they have failed to meet analyst expectations.

Shares of Apple were off $5.70 to $138.02 on Nasdaq after AT&T, the exclusive service provider for iPhone, said it signed up 146,000 iPhone customers as subscribers in the first two days of iPhone sales, well below analyst estimates for sales.
Apple had been estimating sales greater than 500,000, and while the actual number of units sold may have exceeded that where it is really lacking is in the activations. Due to numerous issues with activation and perhaps people not buing the phone to use it as a subscription, they aren't seeing the figures they want.

Still, the product is in its infancy and has a lot of time to make up for it. It's a long trek to that figure of 10 million they want in the next year and a half.