From an apple rep:
Yes, we reset the store inventory at 10pm. There have been a couple nights where it became available just before 10, but on the whole, it won't show available until 10 on the dot, and then...
If your local store has inventory available, you will be able to select the store and check out with the order. If the available store inventory runs out, then the pick up button in your cart will no longer be a clickable option (It will be greyed out), and you can try the process again the next day starting at 10pm.