You can also install Cinelerra 4.2 on Ubuntu from the .debs someone made for Debian.
I wrote it up here - though I was doing it on Maverick and/or Lucid, and haven't tried it on Natty and it seems that there are some potential issues that others had on Natty and/or in loading mkv files (which I don't work with so can't comment on). http://ubuntuforums.org/archive/index.php/t-1673392.html
