Here's a little detail I missed the first time I read today's missive from Apple: you can now check the availability of iPhones at the Apple store nearest you. Simply go to this website, select your state, and you'll get a list of the Apple Stores in that states with a circle next to them. Green means they have iPhones, and red means they are plum out. There is no word on how real time this availability checker is, but it has the potential of saving you some time.
Update: It was pointed out in the comments that this availability status is updated nightly. Therefore you can check to see if your Apple Store has iPhones the day before you go, but they might sell out before you get there (and the page will still say they are available until the next day).