They did that with the MacBook 2 years ago and the internet practically crucified Angela Ahrendts... "how dare they put something on display and not let me buy it!" LOL... yep I said that.
But seriously, I wish they'd cut down on display units... most stores have about 40 phones on display... x 500 Apple Stores, that'd be about 20,000 additional customers who could have their pre-orders launch day.
Do understand the how dare they put something on display and not let me buy it! line but for a device that is so personal as phone, I would never want to buy it with out having scene it. I was think of the new Jet Black, there have been comments that it shows finger prints also you can't judge a new surface like that from photos, you need to see it in person. I guess part of being an Apple early adopter is that you trust Apple that you new product is great.
Shipping display units to stores before the iPhone goes on sale wouldn't take away stock from buyer. Those store will get the display units either way, just a week earlier or later. Store wouldn't need a 40 units, just one or two of each color/size combinations. I do agree that store may not need so many on display, but once that initial rush is over and the phone isn't is short supply that cost to Apple to have more display units is very little.