It is possible, but depends on what you are using as your front end. I am using XBMC and I can specify what audio source to use. I have 2 zones in my house and love it. Zone 1 is my 5.1 surround sound through HDMI. Zone 2 are my ceiling speakers throughout the house.
My HTPC is running Windows 7, so I have zone 2 set as default. Subsonic is running on my HTPC and I have it set to run a playlist. This way anyone can hit play on the remote and music will start playing throughout the house. I have a touch screen pc in the kitchen and normally use this to control music. Subsonic has what they call JukeBox mode so anything I select plays from the HTPC itself
Can even control it with my phone if I wanted to.
If I want to watch TV through zone 1, I simply turn the TV on and XBMC is always running. Just find the TV show or Movie I want to watch, and hit play.
Works great when neighbor kids come over and want to watch a movie while the adults listen to music as the same time.
I believe there is a way you can have 2 zones within Windows Media Center, but I find XBMC more user friendly and flashier.