Will the Sprint iPhone 4S work on ATT?

Yes if it is unlocked. The 4s is a world phone and works on both GSM and CDMA networks. Sprint is CDMA and AT&T is GSM

I do not believe that this is true.

It will not work with U.S GSM carriers.

It will only work for roaming in other countries.

No, roaminggnome is correct. Sprint will unlock the phone for international roaming but it won't work on AT&T or TMobile in the U.S.

Thank you for the confirmation.

I was not 100% sure.