That's a really interesting question. They will definitely receive returns that cannot be sold as new. They will need a way to get rid of them. I bet you will see refurbs, but probably only with a 75 dollar discount.
And what about used iPhones? How easy will it be to transfer numbers and service?
And finally, what would happen if/when they release a 16 gig iPhone. I'm sure lots of people will want to dump there 8 gig and get the bigger storage? What will the used market bring in? Maybe Apple would offer to upgrade people for a couple hundred bucks, and used the purchased stock for a lower priced offering.
I look forward to seeing how this plays out, because the used/refurb thing could be central to Apple bringing a lower cost option to market. If in a year you can get a refurb 4 gig for $300, they will sell a lot a lot a lot of 'em.