When I disable mobile data in Android 2.2, the 3G symbol disappears. Does this revert my phone's signal to 2G? I don't want to lose my signal when I go to the subway, but I don't want to all my apps consume all my monthly bandwidth.
No. This completely disables all data access. Only calls/messages can be used.
EDIT: You might try looking into something called JuiceDefender. The free version will disable your data access when your screen turns off. Good for both battery saving and data-usage saving!