Wifi is always preferred over radio (3G/EDGE/GPRS) so if you want to force cellular data, you have to turn off wifi or un-join whatever networks are in range and/or disable auto-join. There is no failover, so if you get connected to a wifi that has no route to the Internet, the phone won't figure that out and use the cell data. You have to manually get off that wifi before the data will flow over the cellular network..
I often turn off WiFi when I know 3G is faster than certain busy times at coffee shops or when my home ISP is overloaded.
When traveling I've heard anecdotes that even with a wifi connection, some data was billable over cell connection as a roaming charge but can't say if that is still true or was a bug or was user error. Use the switch to disable data roaming if you want to be totally sure only wifi is used.