In a Fedora forum thread - http://www.fedoraforum.org/forum/showthread.php?t=255048 it was found that some makes of hard drives can cause this problem as they take a while to spin up so the system ignores one of them and carries on with the degraded array.
To fix this, apply the following lines to your /etc/rc.d/rc.local file. Make sure you backup the file before doing this just in case and do this all as root;
su
cp /etc/rc.d/rc.local rc_old.local
<b>now using your favourite editor, add these lines to the end of rc.local</b>
sleep 30
mdadm --manage --stop /dev/md0
sleep 5
mdadm --assemble /dev/md0 --auto=md /dev/sd[a-b]1
#mdadm --assemble /dev/md0 -amd /dev/sda1 /dev/sdb1
Substitute sd[a-b]1 in the 4th line and /dev/sda1 /dev/sdc1 in the 5th line to suite your personal setup