hi
I live in Canada and just returned from trip to US using my iPad 3G (purchased in Canada) with AT&T data plan. please see
my post from another thread here which should answer your questions
note: not all Canadian credit cards will work with the AT&T signup procedure. my Canadian Visa (CIBC) did not work. but apparently Canadian AMEX cards do work. and entering your correct Cdn billing address with your Cdn Visa card will also fail (the AT&T signup form on the iPad will fail without a valid US address, it only lets you enter US-style ZIP codes, not Cdn postal codes)
in summary, US hotel address and US-bought VISA gift card will work - that's what I used successfully. but as I said, please read my other post for more info