IMO Apple will setup the system so each iP4 bought before Sept 30 will get a free bumper. So when you go to the store they will cross reference the receipt for your bumper with the serial number of your iPhone that way each iPhone can only get 1 free bumper.
If you have an official Apple bumper or some other case sold in Apples store then you may to be able to bypass the receipt part because your iPhones serial will show it's free case hasn't be redeemed, but thats a BIG IF.
But more so on topic, the point is you want to buy a case now while supplies are still out there vs. waiting on Apple's Priority List for weeks hoping to get a bumper once they have over 1 million orders placed.
Too late. Should have ordered one last night.