assuming you have two available identical partitions to use for the
RAID setup, the way *I* would _attempt_ this is to...
1) make sure I have a grub disk available in case I blow it and can't
boot...
2) create the RAID1 array using the two partitions.
3) migrate my /boot to that new array
4) fix up my /etc/fstab to point to it
5) fix-up my /boot/grub/menu.lst entries to point to the right devices
6) update-initramfs
7) reboot and pray.
this procedure was tested by me several months ago, but I don't
remember it exactly, so you are suitable warned...
if you are already booting to a / on some combination of LVM and RAID,
then that's really the hard part. Its pretty straightforward to get
/boot onto its own RAID1. You can manually install grub onto each disk
that holds the partitions in your /boot array so that if a disk fails,
you can still boot...
please study up on this before attempting as you don't want to blow
it.
A
oh, an alternate procedure might be to go ahead and just make a fs on
one of the partitions, migrate /boot and get that working, then make
that partition into a degraded array (missing disks) and make sure it
boots fromthere and then finally add the second (or more disks) to the
degraded array and then it will mirror and be operational.