There are multiple OS on a computer (e.g. Windows and Linux).
If you want to switch the operation system and are currently using Linux with gnome, I'll select reboot from the main menu, wait until the grub menu shows up, then select windows (which is a menu item configured in menu.lst) and wait until windows is up.

I now want to select the menu item before linux goes down so that I won't need to stay with my computer till grub shows up.

This might also be useful if one does remote management and therefore cannot interact with the grub menu while the computer is booting.

Do get grub to have a menu item preselected and skip the boot menu once,
the required actions are described on 
http://debiananwenderhandbuch.de/administration.html#id1043694 (german language).

Basically, one runs the grub application and enters "savedefault --default=N --once".
A little script would do:

#!/bin/sh

if [ `id -u` != 0 ] ; then
	echo "you must be root!"
else

	if [ ! $1 ] ; then
		echo "Usage: "$0" kernelnumber"
	else
		echo "setting kernel "$1" once"
		grub --batch <<EOT
		savedefault --default=$1 --once
		quit
EOT

	fi
fi

Now I'd like to ask you, to integrate that feature into the gnome main menu.

I now there may be some difficulties as root permission is required, but as reboot itself usually requires root permission, there might be a way.

Thanks a lot,
 AZ