I answered this in a PM, but in case anybody else finds this thread and would like to know how I do it. I use a simple solution which is good enough for casual listening but probably not for expensive high-end systems: A bunch of Sound Blaster Live! and Audigy cards with the kX drivers. The kX drivers let you have three separate outputs from one card, so three cards in my media server gives me 9 outputs. The Windows 7 drivers are in beta (and probably will be forever), but they are 100 % stable for this use, I have been running my newest i5 media server for two months now (24/7) without one single hickup. So that is what I would recommend.
The basic setup is that I have a bunch of used, cheap amps (most of them old home theater receivers, one per zone) in the attic, where the media server is. Short digital or analogue cables from the computer to the amps. And long speaker cables from the amps to the zones. Some are ceiling speakers, some are regular speakers with the speaker cable running down inside the drywall. It's not a hifi setup for very discerning listeners, it's a multi-zone system for a regular family. Since FM radio reception is getting worse and worse in my corner of the world I have two MC running on the server, one for web radio and one for multi-zone music. So each zone can choose either to listen to one of the radio channels I have programmed in or to music they select themselves with either NetRemote or Gizmo. The radio channels are on the IR remotes, using the standard radio channel buttons. But the amps are always on the input from the computer. I use Next Generation IR remote extender, which is a battery with a built in IR to RF converter/sender. Then I don't have to point the remote at a particular spot. I also have a LED diode in each zone showing if the amp is on or not.
In Windows 7 you can name the outputs after what zone they go to, so it's easy to keep track of them. I don't use the synchronize within Media Center, because I have several overlapping zones, and it's difficult to get them 100 % in sync. Even a few ms will be audible. So I have one zone designated as a master zone which then sends to all the amps for "party mode".