Apple has sent pre-order customers a letter informing them that their iPhone 4 will arrive on June 23rd. That date is one day earlier than the official launch date of June 24th.
Dear Apple Store Customer,
You recently received a Shipment Notification email from Apple advising you that your iPhone has shipped.
This email is to confirm that your delivery will occur on June 23rd. Although Apple and FedEx tracking information may currently indicate a later date, you can check the FedEx website the morning of the June 23rd to track your package to your doorstep.
In the event that you will not be available to accept delivery on June 23rd, it may be more convenient to use our pre-sign delivery option by visiting our Order Status website at http://www.apple.com/orderstatus
The Apple Store Team
This is likely an error although the date is mentioned three times in the email.