I've been using the 360 as an extender to play mkv files mostly fine. However larger mkv files with ac3 audio stutter and don't play corrently at all. Files with DTS play fine however.
I started off only using the DivX mkv tech preview, with ac3filter. I've tried various other combinations since to no avail, at the moment I'm on a clean install with the 2 mentioned above and the shark007 pack.