There is an advantage to the 3 year contract (the longest in the world) that is required by Rogers/Fido for the iPhone 3G: monthly cost.
The Canadian 8GB iPhone 3G is only $199 (plus tax), the 16GB is $299 (plus tax).
The cheapest plan available with the iPhone is $67.45 (That includes the SAF and 911 fee), plus tax.
The cheapest voice plan with ATT in the USA (same price points for the iPhone) is $41.24 (39.99 plus $1.25 recovery fee). A mandatory data plan is $30. The ATT web site says the data plan must be matched to an "eligible" voice plan, without providing details. But lets assume that their cheapest advertised plan is eligible. So the cheapest monthly payment you will be able to get away with an American iPhone 3G is $71.24 (2 year contract).By the way, with ATT you also have to pay a one time $35 activation fee.
So the iPhone in Canada (assuming you use 150 minutes or less and 400mb or less and don't want call display) will cost you $3.79 less per month (when the currencies are at par). And as far as I know, there is no activation fee. And although with taxes the Canadian iPhone is slightly more expensive that the US one, when you get a brain tumor, the exorbitant health care costs will be covered by the government.
If you use more than 450mb or 150 minutes, then that is an other story...