Even though this video is a blatant ad for Ford Focus, it nonetheless shows what I have been talking about. The built in stereo Bluetooth receiver allows you to establish a stream on your phone (assuming your phone has a data plan) and then play it back through the car speakers.
As a side note, it would be interesting to do some testing on the interaction between the low-quality of the Bluetooth codec (SBC) and low-quality streaming codecs like Windows Media audio. In any case, the buzz is that SBC doesn't quite cut it, so folks like APT, Kleer, and Qualcomm are creating alternative wireless solutions. Both APT and Qualcomm aim to work over Bluetooth transport, which, in my view, makes them more attractive.