I have successfully gotten my iPhone 4S Verizon to detect and attempt to use the NET10 SIM card that comes with Prepaid NET10 phones.
However since I don't have a independent SIM card that works in Unlocked phones I cannot get this SIM card to work.
I have ordered a T-Mobile SIM card thats coming in the mail on 4/26/12
Once I get this card I will be able to use it in my unmodified iPhone 4S (Verizon, Unjailbroken, Fresh restored)
1.) Restore Phone
2.) Setup like normal and activate on Verizon/Sprint
3.) Turn off location services (if not already off)
4.) put in Airplane mode
5.) take out Verizon/Sprint Roaming SIM card
6.) Put in foreign/other SIM card
7.) turn on WiFi keeping Airplane mode on (important)
8.) Goto Safari and download a APN that corresponds to your carrier from http://unlockit.co.nz/ (free)
9.) Install APN profile (when downloaded)
10.) restart phone
11.) turn off Airplane Mode and wait
12.) turn Airplane mode on for ~10 seconds
13.) turn off Airplane mode
14.) wait until you get service and the title of your carrier in the status bar
15.) congrats you have a CDMA enabled 4S on a GSM carrier!!!!
******This only works on 5.1 (9B179) (Tested on Verizon 2.0.10)********
More info with T-Mobile when I get the SIM card
On iPhone 4S's using CDMA networks don't support the "Enable 3G" button
As you can see here the Verizon-enabled iPhone 4S has the "Enable 3G" on a Prepaid NET10 SIM card
This means that even though NET10 disallows any non-NET10 branded phone to use their prepaid SIM cards the iPhone still detects and it and attempts to use it!!
You can see here that when the 4S is using Verizon with the Verizon roaming SIM in it doesn't have the "Enable 3G" option