You'll end up roaming on one of the carriers in Canada - Rogers I believe.
Have sent a link to your private mail. It may be a more affordable option to buy AT&T's Canada Plan before you go.
AA
I spend time on the north shore of Lake Erie in the summer. I have a Canadian service provider
but on occasion a Nextel (U.S.) cell will expand and overtake my service providers cell and I get a nasty surprise on my next statement.