I'd found a way to shut down the data if that's what you meant by the 3g radio.
You need either Better Cut (or Any Cut) installed:
Long press on the home screen.
Choose 'Shortcuts'
Choose 'Better Cut' (Or Any Cut if that's the case)
Choose 'Activity'
Select 'Data Call Settings'
Select an Icon, choose a name and select 'Create'
Then open the shortcut and it appears to take you to a menu that might be normally only accessible through some uber secret technician master code of awesome powerfullness.
But yeah... just uncheck 'Data Call Enabled' and that will cut off all data communication.
Hope this helps...
Confirmed SMS works too with it disabled.