If you pre-purchase the Collector's Edition, you will have five days after the Guild Wars 2 launch date to apply your retail serial code. If your code isn't entered during the five-day period, you will be unable to access the game until you do apply the code to your account.
I got my boxed CE with the proper code, but I'm not seeing any option to actually... y'know... enter it, anywhere on my Account page. I already put in my pre-order code ages ago but I presume this means I only get the one day head start until I can actually add it.