Will I get charged if I use 3G on iPhone 4?

Like if I'm on the bus and decide to go online, can I use my 3G without being charged? Or can I turn off data roaming and use 3G no charge. Right now I have 3G disabled and am using the "E" thingy.

3g is using the data package you have with your contract. So yes. Just use a little of it at a time. It CAN last for about 4 weeks until your next cycle.