Here's how it works! And it was the same for pre-orders.
When you make your pucrhase through Apple / AT&T they pre-authorize your account then and there but you are not actually charged till it ships (or maybe a day or so after).
So whats happens is the pre-authorization hits your account to verify you have the correct funds in your bank then it sits there till they send the receipt ticket to your bank then your bank will release the funds to the merchant. But Most banks only let a "Pending" charge sit in your account and count against your balance for about 3 days then after the 3 business day if they don't receive the receipt to release funds within that time then it gets dropped off your account and put back into your balance.
But make sure you leave those funds in there cause when they go to set your phone for ready to ship they will push through a full charge to your account and if the funds are gone it means no phone.
So don't worry they will take the money but it wont fulling process 100% until its ships or within a day or so after.
Hope that helped ya.