hey cc, the music stream through airtunes is full digital, you may lose a little through periodic bandwidth restriction when you use your network for other stuff but it's still pretty solid quality. It definitely beats windows media center streamed through an extender like the xbox 360 or another pc in both, stability and quality.
Please keep in mind that audio streams are a joke compared to what airtunes is specified for: as an extender to apple tv and therewith hd video streaming.
If you're into high end, maybe you should look into a good multi zone amp and wire your house in thick copper rather than a wireless stream - but then again, a digital, compressed audio file with 128 bit sampling rate already loses a lot compared to the analog original - so I don't think you lose any additional quality streaming through airtunes. If you have plenty hard drive real estate, I'd recommend sampling your audios in 192 bit and above, use airport express to set up a new network not shared for internet access (use it exclusively to stream music) - your sound should be amazing.
