Dual boot ubuntu 11.04 and fedora 15

You may check this article: Dual-boot Fedora 15 and Ubuntu 11.04 with either side on an LVM partitioning scheme It very detail article. Hope this help.

GRUB boot menu:

Create Custom Layout:

I have dual-boot with Ubuntu 11.04 and Ubuntu 10.10.

I assume that one of Linux OS has been already installed on your PC.

When you start installation, it should ask if you want to add Ubuntu(or in your case Fedora) alongside.

If you choose this option, it will give you opportunity to specify how big partition you would like to get(shrink) for new OS (Fedora or Ubuntu 11.04.)

Then after installation you can change preferences of Startup Manager and specify which OS will be as default