Back from vacation in Virginia Beach, and I thought I'd let all know that the AT&T SIM card I put in my Canadian iPhone 3G has worked flawlessly for the complete duration of my trip.
I had read on different sites that AT&T might deactivate the SIM, supposedly because they could detect that my phone was canadian, but no such thing happened.
So, I definitely recommend using the GoPhone SIM card in the iPhone 3G. As for the iPhone 4, it doesn't work as far as I can tell.
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