From my understanding, Fave Five requires more than having T-Mobile and having a Fave Five plan. You also normally also have to use a compatible phone that supports the software. So I'm not sure even if you unlocked and signed up for a plan that you'd be able to use the iPhone for Fave Five.
And I agree with other posters...it's a show, he makes jokes and false statements all the time, so I wouldn't take this as an indication of much. If he shows his iPhone with extra icons and/or a T-Mobile carrier logo, then maybe you can make this assumption (or that they Photoshopped it...as they're prone to doing)