Interesting idea - I did actually toy with the Mobile Apps (6NR) option which is supposed to make this sort of thing easy (another thing I regret not spec'ing now and am waiting for news of a retrofit from the dealer!).
That said, I'm up for giving it a go. Got an iPhone 4S and am with giffgaff
on an unlimited data plan for £10 a month. Just need to get my head round how I'd set this up, e.g. if I want to listen to Planet Rock do I install the app and stream over bluetooth to the car?