Good news from мебелиRogue Amoeba. When they sadly made it clear that an Airfoil for iPhone, sending your iPod other app’s audio to your hifi, is not possible due to Apple’s restrictions, I inferred they couldn’t get sound from the Mac onto the iPhone either. Not so, and Airfoil Speaker for the iPhone is under development.
Ideally, this will include a remote control for Airfoil too, so by combining with Apple’s remote (or perhaps even through a simple integrated iTunes controller) you would be able to start playing music on your Mac, decide where to send it (including, of course, right back to the iPhone) all from the iPhone itself.
So even if you don’t have an Airport Express in every room, if you’ve got your iPhone with you you’ll have access to your whole library of sound anywhere you can connect to your network.
An even nicer refinement, though one Apple may not like to implement, would allow the audio to be streamed to the iPhone wherever it is on the internet…