Per Apple, signature is at the discretion of the shipper (and that can vary by region as well as designated service level), and this therefore determines whether or not you can pre sign or not.
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My order this time also doesn't have pre signing (but this years pre-order also is a deviation from Apple's past practice where all of their Apple Store pre-orders came from China); some phones (seems to be mostly the non carrier activate later ones) were already in the states and also seem to be set for UPS express (aka overnight) delivery.