If you pre-order CoD: World at War or buy the Collector’s Edition you will receive a bonus on day one of playing online. A pre-order will get you a high-end rifle to play with online while others who just buy the game will need to rank up to get that rifle. Collector’s Edition buyers will get a long machine gun while others will need to rank up to get it. If you just buy the regular copy, you will still be able to obtain both of these weapons but you will need to rank up to get them, just like in CoD4.
Source: GiantBomb.com