Current production of iPhone 4 is entirely dedicated to 8 GB model, only one commercially available. Apple seems to have run out of stock for the iPhone 4 16 GB model as there have been several reports of iPhone 4 brought back to APR or the after sell service, under warranty, and automatically replaced by a brand new iPhone 4S with the same storage capacity.
It remains unclear if this is now the default procedure, but if it happens that you have to bring an iPhone 4 under warranty for service, you might have a night surprise...
