I'm kind of curious to know how many people prefer to pre-order vs just showing up at the store. As far as the 3GS is concerned, this choice could make all the difference.
As you may already know, there will be two separate lines on June 19th: one at 7 am and one at 8 am. The only way you are getting in to the earliest line is if you pre-ordered from AT&T. Everyone else could be stuck for hours, or even come up empty handed. You can also pre-order now from Apple and get the phone shipped to you on launch day. I mean, if you are committed to getting something at launch, it makes sense to pre-order—but I'm willing to bet that a lot of people skip that option for some reason. Consider this a final heads up—although it is already too late for that early line at the AT&T stores.