The term "roaming" in cell-phone lingo means using the phone in an area that is not serviced by your provider. If you are away from you home city but using your phone in Canada in an area with Petro-Canada coverage, then I believe you will be charged their long-distance rate. I would guess (but I may be wrong) that if you use your phone anywhere there is Rogers GSM coverage, you are just making long-distance calls, and you are not actually "roaming".

You would be roaming if you were, for example, in the US and trying to use your PC phone with a GSM carrier there. If there was no roaming agreement between PC and that GSM carrier, then I would suspect you will not be able to make or receive calls (except 911).