Unlocked Iphone 3G for USA T-mobile users. 3G service fee?

I'm planning to get an iphone 3G and planning to unlock it then use it for my T-mobile service in USA. I heard that for AT&T customers, in order for them to use 3G services, they will need to pay extra fee per month. i was just wondering if this is a same case for T-mobile service. because i think razor 2 is 3G and there is no additional fee of using it. let me know if you have any ideas about 3G service with T-mobile.

thx