How can you just pop that sim out when these use Nano sims? I don't know how easily acquired those will be on launch day.
Insert a paperclip and the tray should come right out. I also never mentioned getting a new sim (AT&T should have them in stock anyway just in case issues come up). The sim in the phone you get in the mail won't necessarily be activated yet, it'll activate when you turn the phone on and go through the process on screen. But to be safe, don't turn it on.