The iPhone 4S which was advertised at its release as a "world phone" is now much closer to users, as Verizon and Sprint are planning to offer their subscribers an unlocked version with micro-SIM card slot for GSM networks.
[Update: Following widespread reports in the media to the contrary Sprint has clarified it will NOT be offering the Micro-SIM slot unlocked, so it will not work with foreign MicroSIM cards, although international roaming is possible with the appropriate plan; Verizon is yet to officially comment]
Subscribers who agree to a contract with either of the two carriers will be able to use the CDMA capability of their phone in US, and while travelling overseas (where CDMA networks are scarce) they can insert a SIM card from a local carrier and use the phone over a GSM network.
The only inconvenience is that the number will change as a result, as the new SIM provides a local number. Sprint will be offering an unlocked iPhone 4S from the first day, while for Verizon subscribers the "international unlock" is only available after 60 days of the contract has elapsed.
For AT&T subscribers the situation is even more complicated and expensive. AT&T will offer the iPhone 4S with a locked micro-SIM card slot that will only allow the use of the phone on networks agreed to by AT&T. The cost for calls and data increases in this case, as international roaming services are significantly higher than the alternative of buying a local SIM card with far cheaper rates.
You can win an iPhone 4S by entering our competition - which ends on the 21st of October - here