My neighbour just did this and I am not sure of all of the brand names he used, but in a nutshell, he has wired up every room in the house with in-wall speakers. Each room has a small control panel (about the size of a switch plate) that has a touch pad screen that allows him to turn on/off the speakers in the room, control volume, etc. He really has only one source that drives the speakers. I helped him over the Christmas holidays setup a Mac Airport Express that is connected to the receiver and then setup a new Mac notebook that runs iTunes and the AirPort gets music from.
The only problem that I see with his iTunes system is that the notebook is more or less dedicated to this purpose and you need to use the notebook to select the music to listen to.
Sonus seems to have a good system, agreed it's not cheap.
Howard Stern's producer (Gary Del Abate) has recently wired up his house (it was a new house though) and was the subject of a magazine article (Sound and Vision I think). The article is available online and it shows what he used.
http://www.howardstern.com has the link.