I got it to work using this .asoundrc:
Code:


pcm.dmixs51 {
  type dmix
  ipc_key 1024
  slave {
    pcm "hw:0,1"
    rate 48000
    channels 6
    period_time 0
    period_size 1024
    buffer_time 0
    buffer_size 4096
  }
}
pcm.!default {
  type plug
  slave.pcm "dmixs51"
}
pcm.duplicate {
  type plug
  slave.pcm "dmixs51"
  slave.channels 6
  route_policy duplicate
}



You may need to change the 'pcm "hw:0,1"'line to 'pcm "hw:0,0"', I have to do it like that because of a certain quirkiness of my soundcard (CMI8378).
Using this, I get MP3s in XMMS on all my boxes and can watch a DVD in 5.1 in xine at the same time. just needed to change the 5.1 output device in xine to default instead of surround51.