Besides the US mailing address, the more important issue is that AT&T doesn't provide service in Canada, so you would have to unlock the phone to be able to use it with a different carrier like Rogers Wireless. You could very well be refused warranty service if you take an unlocked phone to Apple (Apple USA, that is... Apple Canada wouldn't even touch it, I think).
If you're concerned about the warranty, then wait until the iPhone is released by Rogers.
Then just buy one and try it out. It takes some time to get used to the virtual keyboard, though. The iPhone is excellent as a web browser and multimedia device, but so-so for many other tasks. But the novelty hasn't worn off yet, so my HTC TyTN is still sitting on the shelf. :-)