(not exactly Airfoil for iPhone — similar, only without the Wi-Fi.)
I’ve posted a few times about ways to use iPhones with Rogue Amoeba’s desktop app “Airfoil” and iTunes streaming.
There’s no great (or official) solution yet for wi-fi audio streaming from an iPhone — but there is definitely plenty of interest still. This blog’s top keyword searches continue to be “airfoil iphone”, “airfoil for iphone”, and “airfoil iphone app”. Until just a bit ago, I totally forgot that Bluetooth was an option.
I haven’t tried any of these devices, but the reviews seem to imply they work well.

