DivX has created a new Media Foundation codec that will allow extenders such as the 360 to play MKVs with h264 encoding and ac3 audio without transcoding. I haven't tried this out myself since my router died and my xbox is no longer networked but people on thegreenbutton are saying it works.
http://labs.divx.com/mkvwin7preview
http://labs.divx.com/mkvwin7preview