no because of the radio in the N1. The radio in the N1 cannot pick up AT&T's 3g frequency.
I believe there is already an announcement or article about how once the merger is completed, tmobile customers will need to get new phones in order to use at&t's 3g (i don't have the link to the source of this).
But the good news is that the complete merger and integration will most likely take around 2 years or so.