Does disabling mobile data revert from 3G to 2G? 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!