Originally Posted by Axcell
It will work ONLY if your phone is unlocked.
Also, you'd have to be on a specific network for it to work as different networks operate on different frequencies. Who is your current provider?
You mean if you're planning to use a Canadian SIM right?
Canadian carriers use the same frequencies as US carriers with respect to the iPhone 3G and above. If you're using the original iPhone, then you're limited to Fido, Rogers, and Seven Eleven (who i think roams on Rogers).
If you want to use Canadian SIM cards and pay Canadian prices, you have to unlock your phone.
If you're using it with your current US provider, be prepared to pay handsomely for roaming, especially data.