I was in Canada earlier this year for a week with my unlocked ATT phone and it was a pain in the ass, most carriers would not sell me a prepaid card unless i got into a contract (which of course i wasn't since I do not live there) and the ones that did offer once required a Canadian credit card to open the account...which of course I did not have either.
If I would have known I would have enabled international data plan form ATT and saved myself the headache (I understand this is not an option for you).
In short I spent a week and could not use it other than in wifi locations.