Spotify has just launched in Australia, which is nice for fans of streaming music, but the times when I most want such a service are when I have extremely limited resources at my disposal - low battery life and limited mobile data quota. So, personally, I don't think I'll ever fully switch over to internet-based music, if only because I won't be able to get to it when my monthly data quota has run dry or I am out of a high-speed service area. If my mobile plan included an unmetered option to sync and store music on my car radio, then I might consider it.
Mokalus of Borg
PS - Also, I'll need a more impressive car stereo for that.
PPS - And that's not going to happen soon.