If you install the bootloader for fedora in the same partition as you install fedora you can just add a chainloader in the ubuntu /boot/grub/menu.lst for fedora and you should be OK.
just make sure you have the partition right. This will hand off the boot to the fedora bootloader and get out of the way. This will avoid any problems with kernel updates etc. I have 4 OSs on 4 drives on my machine and chainload 3 of them from the grub on hd0.
grub uses hda for drive designations. If you have sda drives they are mapped in /boot/grub/device.map
I keep a few backups of that menu.lst and device.map just in case....