Wirelessly posted (iPhone 4S: Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3)
There is currently no software jailbreak or unlock for any model of iPhone 4S. The CDMA (Sprint, Verizon) model supposedly ships with an unlocked SIM slot for *international* use outside of the US, though you'll have to discuss that with your CDMA carrier. If you purchase an unlocked 4S directly from Apple it is configured to only work with a GSM carrier (AT&T, T-Mobile).
I would say if you can afford $750 just buy the unlocked 4S and head to T-Mobile for a microSIM. That way you'll never have to worry about "accidentally" upgrading and losing your unlock. There's a thread in this forum for getting a 4S to work flawlessly with T-Mobile. Good luck!