The iPhone 5 sold by Verizon is unlocked, which means it can accept SIM cards from other carriers including AT&T. You still need to buy a Verizon data plan contract to get the phone at a discount, but after the plan is over you can change carriers easily.
Customers still need to buy a two-year data plan contract from Verizon to get the iPhone 5 at its discounted price of $199.99 for the 16GB model, $299.99 for the 32GB model, or $399.99 for the 64 GB model. For $649.99 for 16GB/$749.99 for 32GB/$849.99 for 64GB you can buy the phone from Verizon with a month-to-month plan.
This policy is new and unusual for a major carrier. Several months ago AT&T began allowing users out of contract to unlock their iPhones, but this involved a complicated procedure. As the AP story points out, "[l]ast year, the first shipments of the Sprint iPhone 4S were unlocked and worked on AT&T, but Sprint later issued software updates that turned off that capability."