iPhone users who are unhappy with service from their carrier, AT&T, now have another reason to make the leap to Verizon when Big Red starts selling Apples darling next month.

Verizon COO Lowell McAdam made it official, announcing today that his company will offer you a $30 unlimited data plan when you buy an iPhone from the carrier. McAdam says he doesnt want to put up any barriers that might keep AT&T customers from making the switch. Although AT&T did away with unlimited plans for new customers last year, McAdam pointed out that many AT&T customers were grandfathered in to their unlimited plans. Without a similar plan on Verizons network, there might not be enough reason for many of those customers to change carriers.

This has the potential to be a huge development in the mobile-carrier wars. Verizon Wireless has the most subscribers, but just a few hundred thousand more than AT&T, which has relied heavily on iPhone sales to boost its numbers. Now its exclusive deal with Apple is going away, and AT&T must contend with its chief rival not only offering the same product, but with an unlimited data plan for new customers and a network that is widely considered to be more reliable.

Despite Verizon’s promises about the capacity of its wireless network, there are still questions out there as to whether it can handle what could be a huge increase in data traffic if it sells several million iPhones this year as expected.

It will be interesting to see if this forces AT&T to bring back some sort of unlimited plan of its own.

Update: So when is “unlimited” actually “limited”? After making the announcement Tuesday morning, Verizon said later that afternoon that the unlimited data plan would be available “for a limited time.” There was no word exactly how long that might be.