3Gs and 3GSs were locked to the carriers in Canada, so you'd have to redsnow\ultrasnow to unlock it to be able to even put a US Sim card into it.
However, for those with iPhone 4 purchased at an Apple Store in Canada (like Mine), they come FACTORY UNLOCKED, so no action needed.
Sounds like for you, you'll have to jailbreak\unlock if you want to use a non-Rogers sim. Safe Travels!