I am a Canadian with an unlocked 4S, and I was planning to buy an AT&T prepaid card on eBay, so I can have a US number when I travel to the States, and not pay Fido a whole bunch of premium charges for texting and roaming. Is there any difference between micro sim for iPhone 4 vs iPhone 4S? The only micro sim cards I can see on eBay at the moment are for the iPhone 4. Any suggestions from Americans regarding the cheapest prepaid US carrier to get a micro sim from? My most common US destination is Las Vegas if that helps.