That's the estimate of one Wall Street analyst, but things might not be that bad

AT&T (T) could lose 40% -- or 6 million of its estimated 15 million iPhone customers -- when and if Apple (AAPL) makes a model of its smartphone that runs on Verizon's (VZ) cellular network, according to a note to clients issued Tuesday by Davenport & Company' Drake Johnstone.

But several factors mitigate against a wholesale migration:

More