I was setting up a new server and wanted to give the convenience of "Setting up software RAID during the installation" using the text-gui a try (Ubuntu Server Hardy 8.04 LTS)
Everything went as expected and really comfy, but after partitioning and setting up the SoftRAID, I ran into the following warning/error message:
I found a few forum/newsgroup entries on the web with the same or a similar error message, but they were heavily outdated and sometimes related to LVM (which wasn't involved in my case).The kernel was unable to re-read the partition table on /dev/md/0 (Invalid argument). This means Linux won't know anything about the modifications you made until you reboot. You should reboot your computer before doing anything with /dev/md/0.
[SOLUTION]
After reading what I found about it, it seems that parted/gparted are having difficulties handling softraid and "think" that there's a problem. So I simply ignored the warning, continued - and installation went smoothly and it even rebooted properly from the RAID system partition.