Installing in text mode doesn't allow you to use fdisk. However it's usually experts that install linux using text terminals so there is a need to have the option to use fdisk (instead of the disk druid that only makes extended partitions), and there is also the need to have a shell available right away. Please consider adding a textpert mode or make the text mode be an expert mode, because now we're forced to use old redhat disks to create the partitions exactly as we want them and then use 6.1's text mode to do the rest of the installtion when there's no proper graphics hardware available.
fdisk is available in the current beta of the next release of the installer.