Wow. I've always wanted to buy an unlocked phone from Apple in Canada, but hesitated because I didn't want to have to travel back to get it serviced.
This is good to know for the future.
Like I said, the official policy is that it's at Apple's discretion, so it's still a bit of a risk.
When I first had proximity sensor issues with my Canadian iPhone, the Apple Store in the US agreed to swap it, but they told me that it would be locked to AT&T, so I drove to Toronto to get it swapped instead.
My replacement had some WiFi issues, which a full restore didn't fix, and I didn't feel like driving to Toronto again, so I asked again at the Apple Store in the US, and that time, they told me that they would be able to maintain the unlock, so they swapped it, and the unlock was maintained even though the iPhone has the standard US part number.
I just returned from a 3 week trip to Asia and successfully used SIM cards from Singapore, Hong Kong, Bali, and Indonesia, so I bet I'm probably one of the only ones with a factory unlocked US iPhone now.