I also had the same problem first but after setting up /dev/cciss/c0d1 in addition to the existent c0d0 and retrying the setup in expert mode grub-install worked flawlessly.
c0d0p1 is one with ext3 mounted to / (including /boot) c0d1p1 is ext3 mounted to /mnt/data c0d1p2 is swap I want to believe that is has to do with me running d-i in expert mode and not adding the additional raid0.
