Just wanted to chime in on the CDMA unlocked GSM discussion...I recently just rushed into a sale on Craigslist for an 'unlocked' 64gb 4s. The sale was quick (guy was in a rush, of course), but I did ask him what network he had been using the phone on. He said T-Mobile, so I assumed it was an AT&T unlocked iPhone. Few hours later, I'm home trying to get my ST AT&T sim to take, and find this Gevey Ultra S sim tray. Turns out it is a Verizon phone unlocked for GSM use with this Gevey Ultra S. Little tinkering around and I got it to work 100%, everything is a go (including mms, imessage, facetime).
But bad news is I paid "GSM" price for the phone, but if I decide to resell it I would have to be honest about it being a Verizon phone and will probably lose some $...I still got a good deal on it, $540 for a mint 64gb 4s with Applecare until March 2013. But like others said, CDMA phones go for less...I'm thinking I could still get $500 though since it is unlocked with the Gevey Ultra S.
...will actually most likely wind up just using this phone as it is working on the straight talk sim card, but I'm a little worried about ios updates, as the unlock requires jailbreak...