I was driving down I-95 today from Philly to Baltimore, and for most of the time my GSM phone (which is on Fido/Rogers out of Canada) was showing that it was on AT&T Wireless as the carrier. As I came past the airport, it switched over to showing T-Mobile as the carrier. A few seconds later I received a text message from T-Mobile (subject is "905") welcoming me to the USA and telling me to dial home use 011- or "+" and the number.
Its nice to see that T-Mobile is looking for non-US phones and letting you know what to do to "call home", but it isn't set to deal with region 1 phones, as to call back to Toronto from Philly on T-Mobile, you definitely wouldn't dial 011 to start the call.
Perhaps someone from T-Mobile will see this and tweak their system so it doesn't send this to Canadian phones when roaming in the USA.
Chris