If it does work, get ready to see your minutes melt very fast, the international roaming fees are quite high. It might be worth it to simply get a prepaid in Canada, depending how much you'll use it here.
I spoke to AT&T Customer Service today and this is exactly what I was told. I am an AT&T customer who has a smartphone and data plan. I was told the following:
1. Buying a prepaid phone in Canada is a better option than a Go Phone due to the burnoff of minutes on International calls - but a Go Phone is a good option if all you are doing is local calls.
2. A better option for me could be to shut off the data plan on my phone, program it for WiFi locally and then buy a package of 80 minutes for $30, which includes international and local calls although the 80 minutes is good for only 30 days.