My route to the N1 is as follows:
Nokia E70 (AT&T) > iPhone 2G (AT&T) > Nexus One (T-Mobile)
I think the biggest factor for my switch was cost savings. Having the iPhone 2G, I was paying $98/mo for 450 minutes of voice + unlimited text and 2G data. On T-Mobile, I only pay $60/mo for 500...