I've never understood why they don't put out a few display units prior to the launch day. It would probably help convince people who are on the fence about pre-ordering, especially if you're stuck trying to decide on a colour. You can never really tell what these things look (and feel) like until you see them in person.
Especially this year with the Air, I think that seeing it in person would win over a lot of people.
But then again, I don't think that getting enough pre-orders has ever been a problem for iPhones. Launch day stock is usually sold out in minutes.