“Thanks for contacting us. I can confirm the Xenoblade Chronicles will be available exclusively for purchase through U.S. Gamestop retail locations, www.gamestop.com, and www.nintendo.com.
However, Nintendo never stated that it will only be available to those who preorder it. Also, the website that you mentioned in your email never stated this either. Preordering simply increases your chance of acquiring a copy of the game at or near launch time.”
So there you have it. Pre-ordering is the smart move, but not necessary.