Well that really depends. If you lunch just for North America then only from US store it will be downloaded. That means most of them are from US region. Sure there are dozens of appstore accounts which are using US store, but they not reside in us. So you'll get traffic from them as well.
Is you app specific to US ? Why you want to lunch just for North America? You would get more exposure if you would not limit just for US.
I've found it easiest to just release the game worldwide for free with iAds. Apple has a list of regions that are iAd supported, but they keep updating the list with new regions so its easier to just release worldwide. Either way you wont be making any money from the regions that iAds aren't supported, but when they update the list, you automatically start placing ads in those regions.