The install process under X will not allow the user to perform fdisk and refresh the Disk Druid information to assign mount points. This is true for all SRM based installs (which must use BSD style labels).
The GUI installer now allows you to use Disk Druid to create partitions on the alpha and will automatically create the BSD bootlabels for the user. If you still want to use fdisk however, you can run the installer in expert mode and select to use fdisk to partition the drives. There are still some significant bugs on the alpha at this point that we are working on, so I would not be surprised if you came across other bugs in the installer.