It's remarkably hard to find straight answers on the web about this... (Yes, I am aware of http://forums.macrumors.com/showthread.php?t=1183812 , but it hasn't quite answered my question about AT&T blocking iPhones.) I have a factory unlocked iPhone 4, purchased through Vodafone New Zealand. I am visiting the USA in a couple of days' time and wish to use my phone with a local SIM on a temporary basis (e.g. AT&T GoPhone or T-Mobile Monthly4G). I do not want to sign a long-term contract. My primary interest is data (Maps, Facebook, email) with the occasional text and call. I start my trip in New York City, but I'm also going upstate where reception is not as good. Questions: 1) Can you buy a prepay microsim from an AT&T or T-Mobile store in the USA? (I think the answer is no.) 2) If I buy a prepay regular SIM from AT&T in the USA, activate it on GoPhone, have it cut to micro size and use it, will AT&T a) detect that there's an unlocked iPhone using their prepay network; b) prevent me from using data on that basis? 3) If I buy a prepay full-size SIM from T-Mobile in the USA, activate it, have it cut to micro size and use it, will T-Mobile have a problem with that? 4) Is there a significant difference in data speed between AT&T & T-Mobile? (I understand yes.) The most useful sources to date: - http://forums.macrumors.com/showthread.php?t=1183812 - http://techniknews24.blogspot.com/2010/11/good-news-for-all-us-tourists-pre-paid.html - doesn't look like a particularly creditable website - http://www.prepaidgsm.net/forum/americas/6756-t-gophone-data-unlocked-iphone-4-a.html - the closest to what I'm after, but my Question 2 has divided opinions - http://www.wireless.att.com/cell-phone-service/cell-phone-plans/pyg-cell-phone-plans.jsp - AT&T's GoPhone - http://prepaid-phones.t-mobile.com/monthly-4g-plans - T-Mobile's Monthly4G - http://www.travelgear.com.au/p/TMOB...paid+mobile+phone+SIM+card+with+USD$30+credit - had I done my research earlier, I would have bought one of these. Alas, delivery won't reach me before I leave.