Slim Devices claims the Squeezebox will play from iTunes: http://www.slimdevices.com/pi_features.html although I've never tried this. You can download their SlimServer software and try it with a software Squeezebox and see how the iTunes integration works.

I've had my Squeezebox for about two years and I really like it. Of the audio outputs I've only used the optical, partially as mine is on a shelf and the thinner cable is easier to hide. The sound is just dandy although I haven't noticed any difference with my new 990 (but the 7700 did help :-) Makes a nice clock too.

The Sonos system seems nice but its Desktop Controller software doesn't support Linux which makes it a non-starter for me.