I just upgraded my Ubuntu box to the latest beta. I ran into the same problem I ran into when I first installed the last release, and I could not remember the fix. So I'm writing it down so I won't forget. Ubuntu disables network socket connections to the X11 server by default. For "security" reasons. (Um, I thought that's what xhost access control was for...)
Anyway, you can fix the problem by editing /etc/X11/gdm/gdm.conf and changing
DisallowTCP=true
to
DisallowTCP=false
