Apple is advertising that they will be available to walk-ins at their stores on March 16, which means each store had a certain number of preorder devices allocated, and then some extras. If it's like how it was last time I did the pre-order online, pickup in store thing, in the morning there will be two lines - preorders get in first, then walk-ins. But preorders should be able to pick theirs up any time that day, second day it goes up for grabs.