http://ubuntuforums.org/showthread.php?t=1195275
GRUB_HIDDEN_TIMEOUT=0The hidden timeout option allows a screen to be displayed without the Grub 2 menu, awaiting input from the user for a given number of seconds. It is available to single-OS computers - if multiple OS's are known to Grub 2, this option is bypassed.
On single-OS computers:
    * The menu will be hidden unless the user adds a # symbol to the beginning of this line ( # GRUB_HIDDEN_TIMEOUT=0 ) and the GRUB_TIMEOUT value is greater than 0.
    * If a background image is designated in 05_debian_theme it will be displayed rather than a blank screen during a hidden menu timeout.
    * For integers greater than 0:
          o The system will pause without displaying a menu for the designated number of seconds. If the user does not press the SHIFT key during the timeout the system will then boot the default OS/kernel.
          o If the user presses the SHIFT key to display the menu, the menu will be displayed for the number of seconds designated by the GRUB_TIMEOUT value unless the user again intervenes.
    * With a value of 0:
          o Unless the user intervenes, the system will boot the default OS/kernel with only a slight delay. No menu will be displayed.
          o The user may force displaying the menu as the computer boots by holding down the SHIFT key.
    * GRUB_HIDDEN_TIMEOUT_QUIET=true
          o true - No countdown is displayed. The screen will be blank.
          o false - A counter will display on a blank screen for the duration of the GRUB_HIDDEN_TIMEOUT value.