Open up /boot/grub/grub.conf and set the timeout entry to -1. Here's my grub.conf as an example:
default=0
timeout=-1
splashimage=(hd1,1)/boot/grub/splash.xpm.gz
title Fedora Core 3 (2.6.9-1.681_FC3)
root (hd1,1)
kernel /boot/vmlinuz-2.6.9-1.681_FC3 ro root=LABEL=FEDORA ide=reverse rhgb quiet
initrd /boot/initrd-2.6.9-1.681_FC3.img
title SUSE LINUX 9.2
kernel (hd1,2)/boot/vmlinuz root=/dev/hdb3 vga=0x314 selinux=0 splash=silent ide=reverse resume=/dev/hda6 showopts desktop elevator=as
initrd (hd1,2)/boot/initrd
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