Well I am a dealer out of NYC and this is what I have been seeing:
I have both VoiceStream and T-Mobile cards (all depends on what our Master Dealer sends)
On R225's I get T-Mobile
On 3390 I get VoiceStream
On N105 I get either TMobile or VStream
Those are our most sold phones (probably because they are free) so I see them all day long. It doesn't matter what SIM is in them or what phone they are in. I can put a SIM in from a phone that said VStream and on a different phone it say TMobile. I think it's just the phone.