Hi,
T-mobile works any where that Bell Canada works. However, they are changing how their data is paid for on September 6th. After that date there will be no more tethered data included in their unlimited offerings. At this moment in time, if you have T-mobile it includes 14 gigs of tethering on the phone. For an additional $5 you get unlimited calling and unlimited text in Canada.
T-mobile will be charging $15 for 5 gigs of tethered data after Sept. 6, but I rather doubt that will apply in Canada. Existing accounts will be grandfathered in.
It is rather a HOOT to me that T-mobile coverage is better in Canada than in USA.
The telcos are greedy beasts.