It turns out Sprint's locking down of the iPhone 4S SIM wasn't exactly a bombshell. Shortly after the iPhone 4S launched, Sprint told Jason Snell of Macworld that the locking policy would eventually go into effect.
According to Sprint, the company will "allow existing customers in good standing to unlock the SIM for international use if needed in the future" if customers explicitly ask for the unlock. That's decent of them, but it also makes me wonder why Sprint is bothering to lock the SIM in the first place. I doubt very many people are looking to buy the iPhone from Sprint and then migrate to AT&T or one of the other US-based GSM carriers.
Happily, this means Sprint customers will not be locked into exorbitant international roaming rates. Instead, so long as Sprint honors the request to unlock the SIM, customers will be able to swap in a SIM from a local carrier and pay local rates.
While that may sound like a hassle, I can tell you from personal experience that it works quite well; I put an Australian SIM in my NZ iPhone during a weeklong vacation in Melbourne earlier this year, and the process went extremely smoothly. Then again, iPhones sold in New Zealand are never locked to a specific carrier, so that definitely made the process easier than it might have been otherwise.