From what I've noticed, there aren't too many shortages for pre-ordered white Evo's. The issue may be that since you ordered it online, your order will literally be processed in the order it was recieved, which means if you were the 101th person to pre-order online yet the warehouse only recieved 100, your S.O.L. I used to work retail for a Video Game Store and this ALWAYS happened to ppl who pre-ordered online, my advice: next time just go to the store.
Don't worry though, word is Best Buy has more then enough for pre orders! Check the tracking number or call their customer service to see when your order will be delivered; give them an excuse like you'll be going out of town and don't want to miss the delivery, see if that works. Good Luck.