Steve,
This is all I have found so far and it definately isn't incouraging:
http://thegreenbutton.com/forums/post/194441.aspx
Reports this:
Due to what I consider a design flaw Vista Media Center or Windows Mediaplayer will only allow you to output audio to the default Playback device. So if you want to play video through MCE and music through Mediaplayer then you will have audio output to the same soundcard.
Someone else suggested that Winamp could select which sound card to use, but Vista may defeat that too.