Ok, made some progress. I discovered that even though my iMac won't boot to the Ubuntu FW drive (and in fact says it can't even read it), once I'm booted into Ubuntu using the install disc, I can then mount the Ubuntu FW drive. So by doing that, I was able to open the menu.lst file. I tried typing in what you said, and I assume the next step is to just save the edited file, but it wouldn't let me save it. Said something about I didn't have those permissions.
Anyway, I copy/pasted the contents of the menu.lst into a text file, which I'm attaching here. Hopefully this works. I'm in Ubuntu right now, and saved this file with Ubuntu's text editor. Not sure this file type is attachable here, but here goes.
Ok, that didn't work. I'll just paste the whole thing in here:
# menu.lst - See: grub(8), info grub, update-grub(8)
# grub-install(8), grub-floppy(8),
# grub-md5-crypt, /usr/share/doc/grub
# and /usr/share/doc/grub-doc/.
## default num
# Set the default entry to the entry number NUM. Numbering starts from 0, and
# the entry number 0 is the default if the command is not used.
#
# You can specify 'saved' instead of a number. In this case, the default entry
# is the entry saved with the command 'savedefault'.
# WARNING: If you are using dmraid do not use 'savedefault' or your
# array will desync and will not let you boot your system.
default 0
## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the default entry
# (normally the first entry defined).
timeout 3
## hiddenmenu
# Hides the menu by default (press ESC to see the menu)
hiddenmenu
# Pretty colours
#color cyan/blue white/blue
## password ['--md5'] passwd
# If used in the first section of a menu file, disable all interactive editing
# control (menu entry editor and command-line) and entries protected by the
# command 'lock'
# e.g. password topsecret
# password --md5 $1$gLhU0/$aW78kHK1QfV3P2b2znUoe/
# password topsecret
#
# examples
#
# title Windows 95/98/NT/2000
# root (hd0,0)
# makeactive
# chainloader +1
#
# title Linux
# root (hd0,1)
# kernel /vmlinuz root=/dev/hda2 ro
#
#
# Put static boot stanzas before and/or after AUTOMAGIC KERNEL LIST
### BEGIN AUTOMAGIC KERNELS LIST
## lines between the AUTOMAGIC KERNELS LIST markers will be modified
## by the debian update-grub script except for the default options below
## DO NOT UNCOMMENT THEM, Just edit them to your needs
## ## Start Default Options ##
## default kernel options
## default kernel options for automagic boot options
## If you want special options for specific kernels use kopt_x_y_z
## where x.y.z is kernel version. Minor versions can be omitted.
## e.g. kopt=root=/dev/hda1 ro
## kopt_2_6_8=root=/dev/hdc1 ro
## kopt_2_6_8_2_686=root=/dev/hdc2 ro
# kopt=root=UUID=43f03931-a1ad-4085-8529-773751727091 ro
## default grub root device
## e.g. groot=(hd0,0)
# groot=43f03931-a1ad-4085-8529-773751727091
## should update-grub create alternative automagic boot options
## e.g. alternative=true
## alternative=false
# alternative=true
## should update-grub lock alternative automagic boot options
## e.g. lockalternative=true
## lockalternative=false
# lockalternative=false
## additional options to use with the default boot option, but not with the
## alternatives
## e.g. defoptions=vga=791 resume=/dev/hda5
# defoptions=quiet splash
## should update-grub lock old automagic boot options
## e.g. lockold=false
## lockold=true
# lockold=false
## Xen hypervisor options to use with the default Xen boot option
# xenhopt=
## Xen Linux kernel options to use with the default Xen boot option
# xenkopt=console=tty0
## altoption boot targets option
## multiple altoptions lines are allowed
## e.g. altoptions=(extra menu suffix) extra boot options
## altoptions=(recovery) single
# altoptions=(recovery mode) single
## controls how many kernels should be put into the menu.lst
## only counts the first occurence of a kernel, not the
## alternative kernel options
## e.g. howmany=all
## howmany=7
# howmany=all
## should update-grub create memtest86 boot option
## e.g. memtest86=true
## memtest86=false
# memtest86=true
## should update-grub adjust the value of the default booted system
## can be true or false
# updatedefaultentry=false
## should update-grub add savedefault to the default options
## can be true or false
# savedefault=false
## ## End Default Options ##
title Ubuntu 8.10, kernel 2.6.27-7-generic
uuid 43f03931-a1ad-4085-8529-773751727091
kernel /boot/vmlinuz-2.6.27-7-generic root=UUID=43f03931-a1ad-4085-8529-773751727091 ro quiet splash
initrd /boot/initrd.img-2.6.27-7-generic
quiet
title Ubuntu 8.10, kernel 2.6.27-7-generic (recovery mode)
uuid 43f03931-a1ad-4085-8529-773751727091
kernel /boot/vmlinuz-2.6.27-7-generic root=UUID=43f03931-a1ad-4085-8529-773751727091 ro single
initrd /boot/initrd.img-2.6.27-7-generic
title Ubuntu 8.10, memtest86+
uuid 43f03931-a1ad-4085-8529-773751727091
kernel /boot/memtest86+.bin
quiet
### END DEBIAN AUTOMAGIC KERNELS LIST
title MacOS X
rootnoverify (hd0,1)
makeactive
chainloader +1