Straight talk unlocked iPhone 4S

Is this in the US?  Which carrier?

Maybe. They are not a supported carrier. If the network is compatible, you probably can, but you may sacrifice some functionality. There is no support for this.

Thank you, they have the best unlimited everything plans as of now.

I've heard you actually can do this but you need a Nokia N71 which you can buy from straigh talk in order to activate the sim. I guess theres a pretty involved process to got it to work but it is possible.

I am in the process now of trying to port my phone number from a Boost mobile account to Straight Talk, and it has been a complete nightmare. Straight Talk customer service had denied i paid for activation, but my bank shows they took $49.15 from my account! NEITHER my boost phone nor my iphone work now!