Changewave looks at how a Verizon iPhone affects the carrier landscape – if iPhone users will exit en mass to Verizon.

Only 4% of Verizon’s customers plan to switch in the next 90 days. In comparison, 10% of Sprint/Nextel’s customers say they plan to switch, as do 15% of both T-Mobile’s and AT&T’s.  While there is some indication that AT&T has a contingent of customers with itchy feet, it doesn’t appear that there will be a mass migration.

But, as the following chart shows, AT&T’s churn rate is its worst ever in a ChangeWave survey.

What’s behind the weakening loyalty of AT&T customers?

First, better than two-in-five likely switchers from AT&T cite Poor Reception/Coverage (42%) as their top reason for leaving, followed by Dropped Calls (27%).

Secondly, the weakening loyalty of AT&T wireless customers is directly attributable to the upcoming release of a Verizon iPhone.

To gauge how many AT&T customers are going to switch to Verizon when they begin offering the iPhone, we asked:

For those who currently use AT&T as your wireless service provider, do you plan to switch to Verizon if-and-when they begin offering the iPhone*

*Note: This survey was conducted in late December prior to the Verizon iPhone announcement.

A total of 16% of AT&T subscribers say they’ll switch to Verizon once it begins offering the iPhone. Another 23% say they don’t know if they’ll switch.

Importantly, current Apple iPhone owners are the most likely group of all to switch, with more than one-in-four (26%) saying they’ll leave AT&T for Verizon.

Note that among all AT&T subscribers planning to switch, two-in-five (41%) say they’ll do it within the first three months of the iPhone’s release – and another 31% within the first year.