a Squeezebox Duet seems to cost £240 and just does audio, an Apple TV plus an Ipod Touch adds up to £370 and does audio and video. I suppose that's not really a huge markup for the extra functionality. The same amount of gear from Sonos would be £500 and only do audio.
If you truly only care about audio, then you can just get an Apple Airport Express (<$100) and stream to it from any laptop you've got around the house. If you want video as well, an AppleTV does the job, and you don't strictly need an iPhone/iPod Touch. Those are just a pleasant addition. For audio, iTunes (PC or Mac) will happily stream to an AppleTV or Airport Express over your network.
Hey, I'm enjoying my pretty walled garden...