Verizon also announced in 2007 that they would open their network to any CDMA device that was qualified in their labs as being compatible with their network. "Any app, any device" was what they touted.
There is no technical reason for any CDMA capable device from working on either Sprint or Verizon. CDMA is sort of the opposite of GSM in terms of locking. With GSM, the devices are locked to accept only certain carrier's SIMs, but any device that can accept the SIM can get on the network. With CDMA, it is the network that is locked, so only devices that are 'registered' by the carrier are allowed. There is nothing preventing them from allowing any and all CDMA devices on their network, other than them denying access to any device ESN not on their list. Given Sprint agreed to this as a settlement and Verizon announced a policy to allow it, it is strange that they are now reneging.
If they were to both honour their obligations and policies, then any iPhone4S, whether purchased unlocked and full price or locked and on carrier subsidy from AT&T or other GSM carrier should be allowed on either Sprint or Verizon. I wonder if Apple agreed to implement some sort of firmware lock where CDMA is disabled if the phone activates with a SIM. That way Sprint and Verizon would be able to say it's not their fault.
I figured that's how it might work, but I wonder why even Apple is saying the unlocked models won't be compatible with the CDMA carriers. I'd really like for some of the news outlets to dig deeper and find out what's really going on.