US carriers selling the CDMA-enabled version of the iPhone 4S will unlock the SIM slot, so international roaming can be achieved by dropping a local SIM into the phone. That's important as, despite Apple branding the 4S a "world phone", there aren't many roaming agreements between the CDMA and GSM networks that the 4S supports …
Annoying about no GSM-to-CDMA roaming
It would be really nice to be able to take a UK iPhone 4S and enable the CDMA secure element so it would roam onto a Sprint or Verizon network in the US.
It would have been really nice of Apple to include the ability to use a CSIM in the SIM slot. I appreciate that the US operators don't allow CSIMs, but China Telecom does, and there's no really good reason for the US operators not to allow it.
I'm sure CDMA roaming will eventually happen when the deals are in place.
This securie element talk is a bit pompous, the CDMA carrier just needs to know your ESN/MEID which is similar to the IMEI.
Don't get too excited
I've worked for Verizon Wireless, and while their "world" phones are, or can be, SIM lock-free, it only works outside of the US. You can't run what Verizon calls a worldphone (CDMA + GSM combo device, like certain Blackberrys and HTC devices) on a US carrier, even if it's got its SIM lock removed.
Preventing certain carrier IDs from being used regardless of SIM locks is super trivial to implement.
Conversely, you can't run an unlocked Euro iPhone 4s on Sprint or VZW either, because those are lacking the CDMA portion. You could run it on at&t, or, at GPRS/EDGE speeds, T-Mobile US.
And lastly, swapping a CDMA-enabled iPhone 4s between VZW and Sprint won't work either, because it's only the GSM portion that will be unlocked, whereas CDMA carriers tie the phone to the subscriber via ESN and MEID (electronic serial number and mobile equipment ID), and they do keep track of these, i.e. only equipment sold through the respective carrier will be allowed to connect to that carrier's network.
All this means is "you can put a foreign SIM into your VZW/Sprint iPhone 4s, removing the disadvantage US CDMA iPhones would have had compared to at&t ones." Don't read anything else into this.
Erm no, the Euro 4S still has the CDMA portion. Maybe it needs to be enabled in software, but after that's done its just a case of VZW et al adding your MEID to their device lists.
I've swapped networks on an unlocked CDMA phone (blackberry) by just giving the new network my MEID.
Hopefully one day Apple and the networks will find a way to make this more seamless.
Metavisor - there's a lot more to it than that. The billing and signalling systems between CDMA and GSM are totally different and there needs to be a mediation system between them. Such systems exist, but only for the CDMA to GSM direction. GSM operators have never felt the need to set up the GSM to CDMA direction since GSM is available in the US. Furthermore, even if the operators did enable it, it's not like your roaming rates would go down. The GSM operators would still charge the same amount regardless of whether you were on GSM or CDMA.
Devaluing the device
Sure you can put a foreign SIM into the CDMA iPhone 4S, but then you lose your phone number when in GSM mode. That is annoying for most people and reduces the value of the phone as it's main point is to be able to be reached at any time. Keeping the carrier SIM card enables you to use the same number worldwide. For pure data sessions, using the local SIM will be vastly cheaper. You'll just be unreachable any time you put the foreign SIM in.
If you're thinking about international call forward to get your calls, note that not all operators allow this and the costs are often as high as roaming rates.