My account (US, MN) with Virgin Mobile is already doing this with "free" streaming music. A hand full of apps (Pandora, iHeartRadio, Slacker, 8tracks and Milk Music) don't count towards the monthly data cap.
Yep T-Mobile does this as well. There is a way for any streaming music service to put in a request to be added to the list, but I'm not sure if smaller companies get accepted or not.
Edit: There used to be, can't find the link anymore...
Edit: There used to be, can't find the link anymore...