Maybe someday. No wireless carrier could withstand millions of iPhones exclusively streaming content anytime soon. I think it's more likely streaming would be used only when you didn't have a local copy available. 32GB+ is probably enough storage for most people to keep the bulk of their collection local. For people with bigger collections I imagine a smarter syncing algorithm could reduce the need for streaming. Sync newly added files, don't bother syncing stuff I haven't listened to recently, sync my most played songs, etc.