How to use a iPhone 4S (that was activated on a CDMA carrier) with a local GSM network?
The answer to this question is slightly complicated and in essence becomes: it depends which CDMA carrier device you purchased and which GSM carrier you want to switch to.

First the facts - there are three "models" of iPhone 4s sold according to the model number on the back cover:


A1431: iPhone 4s (GSM model China)
A1387: iPhone 4s (CDMA model)
A1387: iPhone 4s (GSM model)


People wanting to use models A1431 and A1387 on CDMA networks are basically out of luck as there seems to be no support from Apple to reprogram the device to even work on CDMA. I don't know if it's a hardware issue or just drivers/software, but it's clearly stated in the Technical Specifications as footnote 3:

3. CDMA available only if iPhone 4S is sold and activated for use on a CDMA network.


There is no indication that GSM is not available on CDMA phones, however my experience is that CDMA carriers Verizon and Sprint won't unlock the GSM sim slot until between 30 and 90 days after you have initiated service on that line.

Additionally, I have heard reports that the unlock is smart enough to prevent a GSM carrier that competes with the CDMA carrier from taking over service of the device. So, in the USA - a Sprint or Verizon phone would not be expected to work with AT&amp;T or T-Mobile USA cards, but would be expected to work with any European, Asian, or other non-US based GSM carrier. I do know of many people that have travelled internationally with Sprint or Verizon iPhone 4S and had no problem once they unlocked, but I've not heard that any GSM card works - either while the phone is under contract with the original CDMA carrier or after that contract period expires.