This is information I posted on another thread, but it is applicable here too:
If you failed to read the fine print on preorders, let me sum it up for you:
While GameStop/EBGames and probably every other store recommends that you preorder to ensure you can buy a console on launch day, they cannot reserve a system for you.
The only thing a preorder does is gives corporate a ballpark amount of how many units to ship to a certain store.
For example, if a store only gets one preorder for an item(as is the case with really crappy games), corporate will only send one additional copy of that game, making the count X + 1, where X is their original bulk amount.
Now, in the case of the Wii, Nintendo intends to have 2 million units at launch, which means they could either possibly have higher or fewer yields. If you do the math, it turns out that there will probably be 200,000 consoles for Canada and some regions of Mexico, leaving 1.8 million consoles for the United States.
Halve that to find out how many consoles will be divided among video game stores(GameStop/EBGames, GameCrazy, etc). The rest will go to your Targets, Walmarts, Toys R Us', etc. This leaves 900,000 units for each.
If you figure that there are 4,400 GameStop/EBGames locations and only 700 GameCrazy locations, the worst case scenario would be between 20 and 50 consoles per store, with our best odds being 100+ per store.