The iPhone 4S might look exactly like the predecessor but it packs a faster A5 dual-core processor, an improved camera and antenna. Some owners of 16GB iPhone 4 units might get lucky and instead of replacement get an upgrade to the 4S.
This could all be due to inventory shortages and, according to reports citing multiple sources, Apple Store Genius Bars have been instructed to replace iPhone 4 models with the latest iPhone 4S if the store is lacking white iPhone 4 16GB replacements units. The situation might not apply to all stores but the sources mention those in the U.S. and Canada. The 4S units handed out would be refurbished white iPhone 4S 16GB models.