My method for AT&T, which works:
Enable Cellular Data and MMS on iPhone 4S, IOS 6.1 and up on Straight Talk
(revised 2/4/2013)
Cellular Data
APN att.mvno
Username leave blank
Password leave blank
MMS
APN att.mvno
Username leave blank
Password leave blank
MMSC http://mmsc.cingular.com
MMS Proxy 66.209.11.33:80
MMS Message Size 1048576
MMS UA Prof URL leave blank
Update 2/14/2013 - IOS 6.1 and up on iPhone 4S
1. Close all running apps by double clicking home button to get to multitasking bar (cleans up icons and saves confusion)
2. Eject AT&T or ST SIM from your iPhone
3. Open notes app
4. Press home button to minimize notes
5. Go to Settings>General>Cellular (Cellular will be greyed out)
6. Insert T-Mobile SIM, when <Cellular> un-greys, tap it
7. Tap Cellular Data Network
8. Double press home button to get multitasking bar, select notes from bottom
9. Eject T-mobile SIM
10. Insert your Straight Talk SIM. IMPORTANT! Wait at least 60-70 seconds for HOME to appear at the top left of your screen. AT&T and HOME will alternate, then HOME will stay displayed. If you don't wait, this procedure will fail, and you will have to start over.
11. Double tap home button to get to the multitasking bar, select settings. You should see the <Cellular> settings page.
12. Enter the settings as given above, double check your entries for typos. Back out of the settings screen completely.
13. Everything should work, switch your wi-fi off on your iPhone and test that cellular data works (open a web page or check mail). Also, text someone a picture to verify MMS is working.
14 (optional) Reboot your iPhone and toggle airplane mode on and off, then test cellular data and MMS again to make sure your settings are staying.