I mentioned this in a thread earlier and realised it'd be good to make sure everyone knows about this. I have a friend who ordered Aion when it released last year who missed out on headstart because he preordered too late on amazon. Most outlets have a cutoff point, where they'll still sell you the game, but they wont be able to give you a headstart code. Basically, if you want to buy the CE, and you know where you are buying it, make sure you check the latest date that the headstart code will be available.
I know most people will order well in advance but for those who are putting it off, make sure you check this detail out or you could miss out on headstart.