HT1937: iPhone: Wireless carrier support and featuresLearn about iPhone: Wireless carrier support and features
Currently Being ModeratedMay 4, 2012 12:51 AM (in response to KickerBassMan)
Well it would seem that your iPhone is locked to Sprint and I believe the only carrier who now supports authorised unlocking is AT&T, so it would appear that no, you cannot use your AT&T sim in your Sprint iPhone. http://support.apple.com/kb/HT1937 iPhone 4S 64Gb iOS 5.1
Currently Being ModeratedMay 4, 2012 1:03 AM (in response to KickerBassMan)
The long answer is no.
If and when Sprint ever unlocks the GSM portion of the phone, it will only allow use of a non-USA SIM card for travel.