There were unmet dependencies (libx264-59) for vlc and mplayer in the latest release of Jaunty Alpha. A bug report was filed by several users on launchpad.net
One of the users posted this workaround to get both mplayer and VLC working in Ubuntu (9.04) Alpha 4.
To solve the problem, just get libx264 here:
http://packages.ubuntu.com/us/intrepid/i386/libx264-59/download
Just installed it manually (sudo dpkg -i libx264-59_0.svn20080408-0.0ubuntu1_i386.deb)
I then installed mplayer and vlc using medibuntu repos.
Running:
sudo apt-get install mplayer vlc
Gets these packages working!