The first part is to install Gnome on the Ubuntu server by executing the following on the command line:
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install ubuntu-desktop
You've already done this but I put this here in case anybody is starting with a Rackspace server in its default state.
Thanks to http://www.ubuntugeek.com/how-to-install-gui-on-ubuntu-12-04-precise-server.html for the above.
I wasn't able to get Vino and Gnome working via the command line but FreeNX is a VNC equivalent for Ubuntu. Command line installation instructions are here:
In addition my FreeNX client wasn't compatible with the default 3d features of modern Gnome. It returned “Failed to load session “gnome-fallback” on logging in so I had to run the following line to get Gnome to run in 2d mode:
sudo echo -e "\n#Use unity 2d for client sessions\nCOMMAND_START_GNOME='gnome-session --session=ubuntu-2d'"|sudo tee -a /etc/nxserver/node.conf
Thanks to http://www.dickson.me.uk/2012/06/18/installing-freenx-server-on-12-04-precise-pangolin/ for resolving the above.
The Mac client for FreeNX can be downloaded from here: http://www.nomachine.com/download-package.php?Prod_Id=3834