Thanks! That was the solution for me on ubuntu too. I believe this actually relates to a permissions problem when using VNC. So for any user who wants to do vnc'ing you'll need to (as root) do
adduser theusername tty
...and the users will need to kill their existing vncservers. Log in with a new SSH session (to pick up the new 'tty' group) and start new vncservers.