I moved from Toronto to BC and haven't had any charges for data use while in different places across the country. When we first got here I didn't change my number to a local number right away and didn't have an ISP at home. We used our phones for tethering access to the Internet and didn't incur any additional charges for 'roaming data' use.
The one thing they do ding you on is if you have free incoming, YOU pay the long distance for receiving a call outside of your area.
The data, as long as you are in Canada the whole time, is the same data as if you were at home ... or at least it has been for me on FIDO.