In comments made to MacWorld, Sprint and Verizon have detailed their policies on the international roaming capability built-in to the forthcoming iPhone 4S, with Sprint keeping their current unlocked SIM slot policy in place, while Verizon will default to its current 60 day unlock rule, whereby the account must be in good standing for the aforementioned length of time before the carrier will unlock the phone to allow for the use of cheaper local SIMs instead of relying on more expensive international roaming  plans.
In contrast, AT&T will only offer international roaming through its own rate plan add-ons and will continue its long-standing policy of refusing to unlock its subsidized iPhone lineup.
Update: Sprint has now backtracked from its initial statements and is now stating that the SIM slot in its iPhone 4S will be locked and will require its own slate of international roaming plans, making this the first Sprint world phone to feature a locked SIM slot. As the Sprint iPhone line is being sold with a much deeper than average subsidy for the carrier, the locked SIM slot on the 4S is being seen as one of the main avenues for Sprint to recover the large outlay it made to acquire and sell the smartphone through sales of international roaming plans.
I wonder if this will pull too much business from AT&T. I’m sure they will respond with something, but this could really hurt AT&T with their business customers.