My Iphone 4s 16g and 4s 32g are both unlocked and have this same issue. In iOS 6 the new settings setup meeses with my ability to fix my APN settings to get my data & mms working with my straight talk sim. BUT there is a way to fix it. I dont take credit for this at all, it was sent to me on another post I had but im just going to modify it slightly.
This is thte only way to get data & MMS working on a non-jailbroken iPhone on iOS 6 as far as im aware of right now
1. Open the Notes app (or any other stock app)
2. Go to Settings > General
3. Insert the T-Mobile SIM (can be brand new/unactivated OR an old one that has had service) will bring up the Cellular option
4. Go into Cellular > Cellular Data Network when it shows up
5. Immediately take out the T-Mobile SIM and put the Straight Talk SIM back in while in the Cellular Data Network tab
6. Multitask between Cellular Data Network and the Notes app back and forth until a signal from Straight Talk comes in and the default non-working data & MMS APN are automatically entered. It is important to wait until the default APN settings show up
7. Once the APNs do show up, edit them manually
8. Go toggle Airplane Mode on and off, and then turn your iphone off and then back on
Data and MMS now work fine.... UNTIL YOUR PHONE TURNS OFF OR DIES!
To fix this follow these steps
After Step 8 and you check that your data & mms ARE working. then turn off your phone one more time and then back on again. Check that data & mms yet again. data should work but mms shouldnt. Here is the fix; go into Settings > Cellular > then toggle Cellular Data off and back on and everything should be working 100000% fine now.
MMS Proxy: 188.8.131.52:80
MMS Max Message Size: 1048576
MMS UA Prof URL: *blank*
I can confirm 100% that this method works on iOS 6