I wholeheartedly agree that this merger is a bad idea, especially for consumers on T-Mo's side of the equation. But for clarity, the Nexus S will never "come to AT&T", just like the Nexus One never came to AT&T. (In that it was never sold, supported, or subsidized by at&t.)
But like the Nexus One before it, that doesn't stop Samsung/Google from releasing a version with radios compatible with AT&T's network. And it so happens it would be compatible in Canada, too!
Give it time, it was like 5 or 6 months after the N1 release before we saw an AT&T compatible version of it. I am not surprised we've not seen it yet. And even if we don't, someone's bound to sell the "Canadian" one to us poor souls in the States.