I don't know the name Sonos, but a quick look at the website gives me the impression that their system is proprietary, so you have to use their transmitter and receiver. I can see why you wouldn't go for that.
I know some companies do WiFi receivers that talk to your existing computer e.g. Logitech does the
Squeezebox. I took a quick look at the manual, and there does appear to be iTunes support, but be warned that it can not play DRM-protected files you bought from the iTunes Music Store. (This is what you get if you buy in to Apple's DRM system: I have no sympathy whatsoever.)
edit: there are a lot of them out there e.g. see the Expansys
Media Streamer category. Some do appear to be Apple-licensed to play protected tracks e.g. the Roku products.