Yes, 0710 supports .mkv files, but does not find them...
Just enter the LinuxMCE Adminsite -> Files & Media -> Media Files Sync..
Then on the left, select the videos folder and find your folder containing the MKV files.
You have to manually select Type -> LinuxMCE Video File on all .mkv files and after that press Add to database.
(you do not need to press Add to database for every file, only once for after changin type on the last file.)
Now the .mkv files show up in LinuxMCE GUI.
I hope the automagic scanner is updated with more extensions soon.
.mkv has been added as a filetype to the DB now so UpdateMedia in the next Beta will automatically find them.
In the meantime you can do the following;
mysql -u root -D pluto_main -N -e "UPDATE MediaType SET Extensions = 'mov,avi,mpeg,mpg,wmv,ts,tp,dvd,vdr,ogm,mkv' WHERE Extensions = 'mov,avi,mpeg,mpg,wmv,ts,tp,dvd,vdr,ogm'"
Login to the console under 0710 and use the line above to add the MKV file type to your system... and its done!