I had originally heard that music streaming didn't count against data on the $30 plan, but on Nov 17th I got this text making me think that maybe they changed their mind:
Free T-Mobile Msg: Upgrade to a $50/month prepaid plan and stream music without impacting your data allocation. Visit my.T-Mobile.com, a store, or call 611.
Can someone verify? (I don't really use any of the exempted services)
I received the same message, but I don't think it means anything. Except maybe to say that they reserve the right to remove these recently added freebies (free music streaming, free tethering) at any time.
Ever since they exempted music streaming services in June, I've not had any of that counted against my data cap. Keep in mind that only streaming radio services like iTunes Radio are exempted. Services like iTunes Match, in which you actually download music files onto your phone while listening, does count against the data cap.
I've been running speed tests continuously while driving the last couple of weeks, checking T-Mobile's new wideband LTE and none of it has counted against the data cap.
I frequently check the data usage on My Data Manager against what T-Mobile counts on my data usage, and haven't noticed anything different. According to My Data Manager, I used more than 6 GB of data last month, but T-Mobile only counted 2.2 GB against the cap.