Hi,
I know this topic is slightly old, but I think maybe the next generations can learn from this:
lacloai posted this message long time ago:
"Say, if someone wants to use Ubuntu to run for a file server, and she/he only needs to run with full multiuser with network without GUI. That's impossible. Right? Because you can't change it to telinit 1 sine that is single user mode only."
Of course this is not true, and we should not quickly point people to download some magic "runlevel setup helper" tools. The original problem was to avoid loading GUI on boot. It was correctly answered how to change the default runlevel to level 3 under Ubuntu, but then you should only disable the gdm at runlevel 3 in the config files in order to get rid of the hated GUI

like this:
1.) cd /etc/rc3.d
2.) sudo mv S30gdm K70gdm
This should do the trick. To get more info why you shall do it like this enter the command "cat /etc/rc3.d/README" and you'll get it.
May the GUI also be added to runlevel 2, you can also disable it in "/etc/rc2.d" like in the example above.
HTH,
Mx