I'm still living in the stone age with my "regular" phone on AT&T, but I'm considering my first smartphone, one of them being the iPhone. I would end up doing a LOT of music streaming from work since I can't use my work computer for stuff like that, and Verizon's unlimited data is very enticing. I really wish AT&T would bring back their unlimited plan, since I like having rollover minutes when necessary and most of my family has AT&T. Not gonna be able to stream music all day on just 2GB though...
I like to stream music too but I prefer to do it over wifi since batt life seems to be better than on 3g.