My colleague installed a RHL71 system with two disks, and converted all 3 partitions to software raid after the installation, including root (there was no separate /boot). There was LABEL=/ in /etc/fstab. Label in /dev/sd{a,b}1 and /dev/md0 was "/" for all of these. mkinitrd greps for /dev/md in /etc/fstab and if not found, doesn't enable raid modules in initrd. Thus, if you have only LABEL's in /etc/fstab, you're in for a surprise as raid modules aren't enabled if you don't run mkinitrd -v. Second problem is probably not mkinitrd's fault but if there's some connection: However, even if you run mkinitd --with raid1, and the module gets added, when you boot raid1 module gets loaded but the partitions won't be autodetected and mounting root fails (the classic 09:00 error).
partially fixed in rawhide, now we load whatever personalities exist in /etc/raidtab.