Ya sure you can use prepaid t-mobile sim in the iphone 3g only if it is unlocked one else if the iphone 3g is locked to some other network provider then it wont accept your t-mobile sim until you unlock it ...You can easily unlock your iphone 3g through online providers like OnlineGSMUnlock.com here they are rendering remote unlocking service to unlock all model iphone from network provider ...After unlocking the iphone you can freely use it with any GSM sim ...