Apple has revealed that more than one million customers pre-ordered its iPhone 4S during the first day in which it was available. By contrast, the iPhone 4 achieved 600,000 pre-orders in its first day of availability in June 2010.The iPhone 4S was unveiled last week to a relatively muted response following speculation that Apple would launch an all-new iPhone 5. The iPhone 4S features the new iOS5 operating system as well as a faster processor, better quality camera, improved battery and new voice recognition functionality.