Not necessarily. The iPhone 3G is different. No serial numbers on the actual products makes selling used/returned/etc merchandise easier without being sued.
Apple send all returned iPhones back to the "depot." Once at the depot, Apple sorts which ones will be used for warranty replacements and which ones will be sent to China to be recycled. This process couldn't be done with the original iphone.
Also, why else does Apple advertise the products being more green. It's because they have to buy less parts and recycle the ones that meet or exceed their criteria.
No serial numbers? All iPhones have serial numbers. Stop making stuff up. You have no idea what you are talking about.