I can confirm this doesn't work.  There appears to be a regression between Ubuntu 9.10 and Ubuntu 9.04 when installing to a software RAID 1 according to the directions in the Ubuntu Server Guide (https://help.ubuntu.com/9.10/serverguide/C/advanced-installation.html).

==Repro case (as above and in the Server Guide)==

1.  Assume two physical devices

     - /dev/sda
     - /dev/sdb

2.  Boot from ubuntu-9.10-server-amd64.iso

3.  Select defaults for [keyboard, time, configure via DHCP, etc]

4.  Create partition tables on /dev/sda and /dev/sdb

5.  Create two partitions

     - 20 GB, /dev/sda1, physical volume for RAID
     - 20 GB, /dev/sdb1, physical volume for RAID

6.  Select "Configure Software RAID", "Create MD drive", "RAID1" using "/dev/sda1" and "/dev/sdb1"

7.  Select RAID1 device #0 partition ("/dev/md0"), use as "Ext3 journaling file system", mount as "/ - the root file system"

8.  Continue installing base system, mostly just selecting defaults [no swap, write changes to disk, etc]

9.  When asking where to install GRUB the default is "/dev/md" and that is what worked for 9.04 (saying "/dev/md0" doesn't seem to work for 9.04).  The error is "Executing 'grub-install /dev/md' failed.  This is a fatal error."  I've tried:

     - /dev/md   => "Executing 'grub-install /dev/md' failed.  This is a fatal error."
     - /dev/md0  => "Executing 'grub-install /dev/md0' failed.  This is a fatal error."
     - /dev/sda  => "Executing 'grub-install /dev/sda' failed.  This is a fatal error."
     - /dev/sda0 => "Executing 'grub-install /dev/sda0' failed.  This is a fatal error."
     - (hd0)     => "Executing 'grub-install (hd0)' failed.  This is a fatal error."
     - (hd0,0)   => "Executing 'grub-install (hd0,0)' failed.  This is a fatal error."

All of them yield the same error.

==Notes==

Ubuntu 9.04 uses legacy GRUB (0.97) while Ubuntu 9.10 uses GRUB2.  The following is the working legacy GRUB configuration the 9.04 installer creates.

############### menu.lst #################

## default num
# Set the default entry to the entry number NUM. Numbering starts from 0, and
# the entry number 0 is the default if the command is not used.
default	0

## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the default entry
# (normally the first entry defined)
timeout	3

## hiddenmenu
# Hides the menu by default (press ESC to see the menu)
hiddenmenu

## ## End Default Options ##

title	Ubuntu 9.04, kernel 2.6.28-11-server
root	(hd0,0)
kernel	/boot/vmlinuz-2.6.28-11-server root=/dev/md0 ro quiet splash
initrd	/boot/initrd.img-2.6.28-11-server

title	Ubuntu 9.04, kernel 2.6.28-11-server (recovery mode)
root	(hd0,0)
kernel	/boot/vmlinuz-2.6.28-11-server root=/dev/md0 ro single
initrd	/boot/initrd.img-2.6.28-11-server

title	Ubuntu 9.04, memtest86+
root	(hd0,0)
kernel	/boot/memtest86+.bin

### END DEBIAN AUTOMAGIC KERNELS LIST

############## end of menu.lst #####################


Huuanito--You might try using Intel Matrix RAID (or whatever version of fakeRAID your mobo supports) at least for the /boot partition.  You would need to use the DVD or server CD (e.g. ubuntu-9.10-dvd-amd64.iso or ubuntu-9.10-server-amd64.iso).  The desktop CD does not include dmraid so any fakeRAID partitions won't show up.

Also note that choosing LILO instead of GRUB on 9.10 works perfectly (no errors and system is bootable).  LILO installs to /dev/md0 by default.