Just spent some time with At&t and Cingular's customer service going over a variety of plans to help eliminate costly roaming fee's and extra charges for using a U.S. cell phone plan while in town. Also looking to keep those costly hotel charges for local calls down to a minimum.
Found the best deal at att....you have to have their national plan at 39.99 a month for 450 minutes. Then before your trip add on the option of canadian coverage for 9.99 a month. No roaming, no extra fee's besides the one listed above....any call you make locally or to the U.S. go against your minutes on the plan.
And after you return you can cancel your canadian coverage....and re-apply at any time.