If it didn't get automatically created for you, then you can make a
new one using mkinitrd. Don't forget to check out
/etc/mkinitrd/{mkinitrd.conf,modules}. You may need something like:
ext3
ext2
in the modules if you need ext3 on boot.
% mkinitrd -o /boot/initrd.img-... /lib/modules/...
Andrew.