Just how many subscribers have ditched AT&T since the carrier lost its exclusive hold on the iPhone to Verizon

AT&T is mum on specific numbers, but a top executive has offered this hazy assessment: the company isn't surprised by the level of defections.

Speaking yesterday at a Morgan Stanley technology conference in San Francisco, Ralph de la Vega, head of AT&T Mobility and Consumer Markets, said his company hasn't seen any surprises in customer defections and that "everything is pretty much within our expectations," according to a Webcast of his speech.

The problem with such a statement is that AT&T's expectations are unknown: did AT&T envision a mass exodus, a trickle, or somewhere in between

De la Vega did address the question of whether most of the impact would be felt in the first quarter due to immediate defections following the Verizon iPhone's launch in early February. He said it's a bit early to tell right now because the "situation is volatile....it changes from one week to the next." However, he said, the general expectation has been that defections would start to trail off after the initial quarter of Verizon iPhone sales.

On the plus side, de La Vega pointed to AT&T's family and business plans as being "retentive in nature," which would point to fewer subscriber losses among those gorups.

Some initial reports and rumors have pointed to early sales of the Verizon iPhone as a bit "underwhelming," noting that the crowds at the stores on launch day were smaller than expected. However, preorders were seen as quite healthy because Verizon reportedly sold out of its stock.

Assuming iPhone 4 sales haven't been quite as hot as Apple and Verizon had hoped, CNET editor David Carnoy has proffered various reasons, including increased competition from other smartphones, subscribers who don't want to break their existing contracts, and uncertainty over when and if Verizon may get the iPhone 5.

Reports this week predict that Verizon will eliminate its unlimited $30-per-month data plan in favor of tiered pricing. Such a switch could happen this summer, which would tie in with Apple's launch of the iPhone 5.