You can use the IPhone with T-mobile is the Iphone is unlocked to use with other carriers other than AT&T. Go to the t-mobile website, they have all the phones listed that they have for offer. There will be many phone coming out soon to rival the IPhone.
Ya sure you can use iphone or any other GSM phone with t-mobile sim only if the iphone is unlocked one or else already locked with t-mobile else if the iphone is locked to some other network provider then it wont accept t-mobile sim until you unlock it .You can easily unlock your iphone through online providers like WickedUnlock.com here they are rendering remote unlocking service to unlock all model iphone from network provider ..You first check that whether the service is available for your country and network provider .If yes then pay for it and unlock your iphone then freely use it with any GSM sim ...