Is McKay being Canadian a retcon for the Atlantis premiere?

It is clearly established in the first episode of Atlantis that Rodney McKay is Canadian. He wear the flag, and pronounces the last letter of the alphabet as 'zed', and this enables Shepard to remark he is Canadian explicitly. This was obviously deliberately done to allow the main protagonists to be international rather than a recurring roles, since Weir, Sheppard and Ford (and Sumner) must all be American, being pre-existing characters or military.

Was there any indication in SG-1 what McKay was Canadian, or was it invented for Atlantis?