third-party carriers

Hello folks, I am looking to buy an accessible phone and of course the iPhone is on top of my list. However, I would like to get the iPhone 5 unlocked. From what an Apple sales rep told me they are not selling unlocked models yet and he was not allowed to give me more information. From your experiences when are the unlocked versions usually released? I would prefer to get a GSM phone because I would like to use my phone if I travel out of the nation. What third'party carriers run on the GSM network? I am looking at Virgin Mobile but from what I can gather they are a CDMA network. Thank you so much for the help. Louis

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AT&T and Simple Mobile

AT&T has a GSM network but I'm not sure if they have prepaid plans. I used to use my GSM iPhone 3GS with a network called Simple Mobile. According to their website, you can bring any unlocked GSM phone to use on their network. I read articles about being able to use the 3GS and 4 on Simple Mobile but I'm not sure about the 4S or 5. It's not a complete answer but I guess it's a start.