However, there are couple of work-around. The one that I found easiest is here:
1. During installation of Fedora 15, keep the Grub on the root of Fedora and not on MBR. This way it will not overwrite Ubuntu's Grub.
You will see below screen at the stage when Fedora asks where you want to install Grub. Click on the Change device button
In the pop-up windows "Where would you like to instll the boot loader for your system?" select the "First sector of boot partition /dev/sda[x]. Here [x] is the partition where Fedora is being installed. Click on OK and then Next.
It will start installing Fedora on your PC. Now there isn't much for you to do. Fedora 15 will start the installation process which will take couple of minutes.
2. Now, once installation is finished boot into Ubuntu and run this command in terminal:
sudo grub-update
It will update Ubuntu's Grub adding Fedora to it.
3. Now, restart PC and you will be able to boot into any distro of your choice.
Source: http://www.muktware.com/articles/25/2011/1231
