I tried to install Ubuntu natty 64bit server on a machine with 4 HDDs. During the setup I partitioned them and assembled a RAID6 array. The very last step of the Ubuntu installation failed. Grub basically said it cannot read the filesystem on any of the HDDs and it could not install itself into the MBR since there is no MBR. This is because natty apparently uses GPT (GUID Partition Table) as the standard partition table format now, instead of the old msdos format. GPT has no MBR, and this is where grub fails.
A workaround for this problem is to boot from CD or USB into Ubuntu Live and use gparted to create an empty msdos partition table on each disk. After that, re-run the Ubuntu server installer and partition the disks (be sure not to recreate the partition table) and after the installation is done, grub will install itself into the MBR of each HDD without complaints.
Another approach would be to create a small separate partition for the grub bootloader code and set the “BIOS Boot Partition” flag on it using gparted or parted. I haven’t tested this myself.