you asked the right guy here. i was at las vegas fashion show mall for iphone 1, valley fair for iphone 3gs, 4, and 5 (4s was mail in). let me tell you, on every single release, you could simply walk into Apple store around 11am on the release day after all the lines are cleared, to buy your phone. They never ran out of stock. There were at least 1000 people for the iPhone 4 line (longest) and they didn't run out of stock until the afternoon of release day.
I can safely tell you that, if you don't care which size/color you want, you can most likely get a phone by walking into the store around 11am. Don't go earlier because the line takes about 3 hours to clear. It moves extremely slowly because every customer needs to be activated.
I think you are 90% guarantee to have AN iphone 5s on launch day if you go at 11am. Why not 100%? Well, this year they DO have less inventory than before and they don't have online order, so we might actually run out. (Unlikely though.)
I personally will go at 5~6am, because what I found is that most people come either the night before, or between 6~8am. Very few people come to join the line between 12am and 6am, so if you come during that time you are not really getting ahead of too many people.
