The partman in the server installer from the daily distribution 20100419.1 is even more erroneous.
I decided to simplify matters by installing a small extra IDE/ATA disk for the system
keeping the 3 x 2TB SATA disks for manual configuration (as soon as partman is raid
ignorant and buggy).
The installer correctly identified the disks as
I created /root, swap, and /home on sdd, leaving 3x2TB disks unformatted.
Then during the final stage it reported "installing grub on MBR of sda" (leaving me no
options; why? I have not even touched sda)
Not surprisingly, the server cannot boot...
And I have not yet started testing what I wanted.