This appears to only work with Debian based linux. Try this out, if you don't already know what it does, just take a guess. I wont tell you.
Bring up your terminal and type "apt-get moo"
then just type "apt-get" by itself. You'll get a usual help file, but the last line meets up with the previous easteregg.
OK, ok.. it's kind of lame. But it just goes to show, not everything has to be serious in computer science. Even programmers like to have fun.