I am using Ubuntu 11.10 (Unity) as Guest/Host using VirtualBox. I don't want a Unity on login and want to directly get to a shell in the Guest after the boot. What changes have to be done?
Unity is a shell for Gnome. Unless you've setup automatic login, then Unity does not automatically start. Do you mean that you don't want to start the display manager, and get a commandline interface instead? In that case, simply remove lightdm and gdm. You can just reinstall it later if you want to.
If you don't want to use the GUI, installing the server version is the best option as it will only install the needed files.
If you want to use the GUI occasionally, Jo-Erlend Schinstad's answer, uninstalling *gdm is a great solution.
If for some reason you don't want to uninstall the gdm, then you can prevent it from running at boot editing your /etc/init/ligthdm.conf file, or the equivalent file if you are using another dm. Just change the line that reads:
start on ((filesystem and runlevel [!06] ...
start on ((filesystem and runlevel [!0123456] ...
This condition will never met (the computer is always in one of those run-levels) and it will never start.