Well, being perhaps the first Canadian car to venture into the US, I can tell you that the 3G works seamlessly on both sides of the border.
i also noticed, after Doug_G's suggestion, that in a spot of our island that has no Rogers reception, but decent Telus reception, the car gets good signal. Of course, this may simply mean that they are using AT&T and it is roaming for all of us in Canada. They must have a corporate deal that gives them a better rate that individuals would get for roaming.
I hope that they just let us choose our own plans and install our own SIM when the one year free starter arrangement runs out.