Hmmm...if only it were that easy. When my SIM isn't in the phone, the "Cellular" menu under General in Settings is grayed out and it says "No SIM" to the right of it. If I can't get in that menu, I can't get to the Cellular Data Network submenu.
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.