I too am in a similar situation - however my house is still under construction.
My plan is to run a single Mac Mini as my media server. It will run Plethra (in addition to iTunes (hosting the music) and Plex (video, etc - unrelated tot this)).
I am going to have 6 stereo zones, all terminating in my utility room.
Depending on the timing, and the cost/performance of the Plethra hardware, it would be perfect for me. With it, I understand that it will take care of the switching and output 8 different zones. Under this setup it would look like: Mac Mini > Switch > Plethra Hardware > Amp > Speakers
If the Plethra hardware is not ready to go, or is otherwise not well received, I am going to use an Airport Express for each zone. You can pick these up on Kijiji / Craigslist for next to nothing ($30 or so, if you're patient). I am going to have each of them in the utility room, hardwired to a switch (which I need anyway for the Cat6 runs to other rooms of the house + I've had minor issues with the Airport Express over wifi).
Under the Airport Express setup it would look like: Mac Min > Switch > Airport Express (x6) > Amp > Speakers.
For either solution you obviously need amplification. I have been messing around with one of these: http://www.amazon.com/LP-2020A-Lepai-Tripath-Class-T-Amplifier/dp/B0049P6OTI and a pair of speakers and I'm extremely impressed. Perhaps if you're a real audiophile you may want something more high performance, but for my needs it will be fine in every room except the home theater, where I will have more proper setup. I considered various multi-channel amps, but for the money, I don't think they add the necessary value. If anything, the real selling feature of them is that you don't need 6 (or 8) individual amps and the associated cables messing up your cabinet.
The total cost of each zone under the Airport Express setup is about $50, plus speakers. It remains to be seen what the Plethra box will cost.
My wife is terrified that this is going to be far too complicated for her to use...but if it works the way it should, that will all run in the utility room, and she'll be able to control everything she wants from her iPhone/iPad and have no idea that