Currently anaconda forces the user to partition a disk and install to /dev/hda1 or similar. When running in a Xen environment (where /dev/hda refers to a LVM volume in Dom0) or a SAN (where /dev/sda refers to the SAN equivalent of a LVM volume) there is no need to create a single partition on each "disk". Using partitions just makes the process of extending a filesystem needlessly complex. Steps to Reproduce: Try to make /dev/hda the root filesystem instead of /dev/hda1. Additional info: The boot loader also needs to be changed to support this. pygrub in CentOS 5 doesn't support a non-partitioned disk and I presume the Fedora package doesn't either.
We don't support installing to raw disks because that's the sort of thing that BIOSes don't boot. If you did it in Xen, then you'd end up breaking when you want to migrate to another virtualization technology later.