The don't officially support iPhones with goPhone, but it will work. My iP4 an unlocked unit and it worked fine. I updated the APN to use the wap.cingular settings and it was all good. You don't need to be jailbroken. Just use http://www.unlockit and select AT&T or Cingular for the APN. I'd suggest doing this before you leave Canada and select the option to email the APN to yourself. Then when you get the States, just open the email and install the APN. You could also use Apple's iPhone Configuration Utility (free download) to manually tweak the APN yourself if you are bringing a laptop with you. (I just realized you only mentioned the JB allows you to edit the APN on the phone, so yes, that would work too).
Originally Posted by jfmezei
TMobile has cheaper data options but the speeds will be slower as you will only use EDGE. When I last went to the states I got an ATT SIM, chopped it down and all was well. If you go to an AT&T store to get the SIM and activate it, they will ask for your phone and IMEI. Tell them you left it at the hotel and they can activate with a dummy IMEI.
Unlocked Pentaband iPhone 5 (AWS), unlocked iPhone 4
Unlimited airtime, Unlimited CAN/US long distance, Unlimited SMS to CAN/US wireless numbers
2500 Call Forwarding minutes to CAN/US numbers
Google Voice for visual voice mail with message transcription, conditional greetings, unlimited messages (vs 35 message cap), remote retrieval from any PC or phone, no auto-purge after 10 days and most importantly no $7-$8 charge.