What I learned DID work was just popping the SIM out, going to the settings screen, popping it back in, doing the app change trick, and then changing the settings as needed.
Basically, follow http://wiki.howardforums.com/index.php/Straight_Talk_iPhone#... but don't use a T-Mobile sim card, just pop the ST one back in.
This is iOS 6, remember. I think the instructions you linked to on the HF wiki were written for iOS 5, since they talk about General -> Network -> Cellular Data, instead of General -> Cellular -> Cellular Data...that menu got renamed in iOS 6. They no workie, at least without a T-Mobile SIM.