Did this hack myself a month or two ago - I go the XBMC route (openElec) on the Pi with Airfoil for Windows to stream Spotify. That combination is finicky - sometimes requires one or two connects. When spotify releases a libspotify compiled for the right ARM architecture (something about floating point?) I'm going to run http://www.mopidy.com/
The disadvantage of the headphone port of the Raspberry Pi is that it used to put out absolutely awful sound, but that may have improved. Using shairport also has bugs - the audio output from the script sometimes has pops and after about 10 or 20 minutes will randomly disconnect.
That's really cool! I want to play around with getting this to play music directly from the internet somehow. I tried XBMC at first but I didn't like how much input it needed to get going and how flaky the results were.
You're right that the headphone port is (still) pretty crappy. I've got the model B 512MB revision and I still hear a lot of static.