Windows 8 has a new bootloader that's a bit different than what you may have seen before, and after installing, it will give you the option to boot Windows 7 or 8 but not Ubuntu.
I don't know of any way to stop Windows 8 from installing that bootloader, so you have to install Windows 8 to whichever partition you set aside for it, then boot up any linux live environment and reinstall Grub to your first partition (Which is either 7 or ubuntu, depending).
Once you boot up in the linux environment, browse to your existing /boot/grub/menu.lst file -- it should show entries for ubuntu and windows 7. If you have changed the partition order installing Windows 8 (e.g. changed the partition number where ubuntu resides) you need to update the line root(hd0,0) accordingly, where the first number is the harddrive number (0 if you only have one) and the second is the partition number -1 (sda1 = 0, sda2 = 1, etc...)
You can create a third entry for Windows 8 in exactly the same way as windows 7, changing only the rootnoverify line to apply to the appropriate partition where Windows 8 is installed.
Once that's all set, go ahead and reinstall grub to your primary partition by opening up a terminal as root and running
then at the grub console type
my grub config for this looks like this:
# (0) Arch Linux
title Arch Linux
kernel /vmlinuz-linux root=/dev/sda3 ro
# (1) Windows 7
title Windows 7
# (2) Windows 8
title Windows 8