According to iPhoneInCanada.ca, to make stocking and inventory easier, the iPhones in Apple's Canadian retail stores will arrive completely unlocked without any SIM card. Once a carrier is chosen and a SIM card is placed in the device, it will then be locked to that carrier. So what happens if you put in a SIM card from a carrier other than the four in Canada? The simple answer: we have no idea!
While we're not entirely sure what this means, it does present some interesting ideas. The Canadian iPhones could only recognize the four possible carrier's SIM cards, but may also be entirely unlocked. It'll be interesting to see how people that want to use the iPhone on unsupported carriers will take this news.