General :: Create Custom Terminal Boot Mode Under Ubuntu?

I wonder if it would be possible to create a custom boot menu entry in grub under Ubuntu (11.04), that boots the system into terminal instead of the login screen.

The effect should be something like the root shell in (rescue mode) boot menu entry, one that gives me access to a root shell or a less privileged shell, without the need to select "root shell" explicitly.

If possible, I also would like to customize the shell started by, most preferably by specifying a shell script to run so that I can start something other than bash, like vim.

My intention is to create a "fast" boot mode to give me a usable system within seconds (to take notes, for instance) without needing to start GUI. Therefore, it is preferred that the silent option is turned on.

Note: There is a requirement: that no existing feature of the system is removed. That is to say, the gnome desktop (and Unity) should remain intact, and is only disabled when I choose this mode.