With previous versions of Ubuntu, you could specify mount points for various hard drives and partitions during the installation process. This was useful, because I have a backup hard drive that I set to mount at /media/backup. When installing Ubuntu 11.04, I can see the box that I used to type this info into, but it doesn't allow keyboard input and I can only choose the pre-defined mount points (e.g. "/" and "/home").
This was not intentional -- it's actually a bug in the installer that was noticed too late to be fixed in time for Ubuntu 11.04.
There is a workaround -- you can type the mount point you want ("/media/backup") in a text document, then copy and paste the text into the box in the installer.
Update -- This has now been fixed. I doubt they'll re-release the Ubuntu 11.04 disk image, but at least we shouldn't have to worry about this problem in 11.10.