the iphone will be able to work with t-mobile. if want to use it on t-mobile, you'd have to unlock it or try to find an unlocked one. Either way, when/if you get it, you'd have to adjust the internet settings for the email and browser to work.
If the Iphone is for cingular, than that means it uses a simcard. Also Tmobile uses simcards, so does that mean that if you have Tmobile you can use the iphone.