My new unlocked 4s is in action here in Canada. I went to a Virgin Mobile kiosk in a mall here in Welland and got a pay as you go plan set up, the sim card installed, and unlike my experience with Yes Optus in Australia, it worked right away.
So now in Canada, I have a new phone number, can SMS, and most key, have 3G data wherever I go (and where it is).
The other gain here is that I can swap in my AT&T sim when I go south of the .ca border and be back on their network– and I don’t have to renew my contract with them when I got the new iPhone (though it cost much more; it ought to make me more careful with it!)