pList editing in iOS 5 and restoring with iBackupBot on a non-jailbroken iPhone has never worked for me. Something always resets the APNs to the default "phone" one. It does the same thing in iOS 6 too.
The only way I can get data & MMS working on a non-jailbroken iPhone is by:
1. Opening the Notes app (or any other stock app)
2. Going to Settings > General > Network
3. Taking the Straight Talk SIM out and putting in a T-Mobile SIM (can be brand new & unactivated. mines is)
4. Go into Cellular Data Network when it shows up
5. Immediately put the Straight Talk SIM back in
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.
7. Once the APNs show up, edit them manually
8. Go toggle Airplane Mode on and off, and reboot the phone
Data and MMS now work fine.
Cellular Data
APN: att.mvno
Username: *blank*
Password: *blank*
MMS
APN: att.mvno
Username: *blank*
Password: *blank*
MMSC: http://www.mmsc.cingular.com
MMS Proxy: 66.209.11.33:80
MMS Max Message Size: 1048576
MMS UA Prof URL: *blank*
I can confirm 100% that this method still works in iOS 6 GM.
Google "Straight Talk iPhone HowardForums Wiki" and what I explained above is on that page too. The entire site is down as of this post so I explained it as best as I could.