I installed 5.5 with a 300GB RAID 1 partition (boot is also on this partition).  It booted up fine the first few times until after I used a Live CD and accessed the array, and it became named /dev/md127 for some reason.

Now whenI boot into CentOS I get a kernel panic and different errors, once I got "invalid superblock", even though the array is fine (it didn't happen again, probably because I was sure to dismount and stop the mdadm array properly).

Here's what I get when booting (the /dev/md127 is the problem I think):

I have tried specifying the root with UUID= with /dev/md0 and /dev/md127 without any different result. I'm really getting frustrated and stuck.  I also wonder if the issue might be that I used a very new 3.1.2 mdadm to access the array on my boot disc, could that have caused some incompatiblity problem with a different and likely older version of mdadm that Centos uses?

Solved

I think that when I accessed the raid array and it became md127, this screwed up all the initrd's or something else.  I don't know how to explain it.

I got the idea to use single user mode and it booted fine.  Then I just made an initrd:

mkinitrd /boot/yournewinitrd `uname -r` and make sure you boot with that initrd, I was able to boot again.  This should serve as a warning for anyone booting with RAID 1.  DON'T ACCESS THE RAID ARRAY WITH A LIVE CD UNLESS YOU USE THE SAME MD DEVICE.  I really think this is a Centos and mdadm bug in one.